Molecular evolution in bacteria: cell division

Evolução molecular em bactérias: divisão celular

Abstracts

Molecular evolution in bacteria is examined with an emphasis on the self-assembly of cells capable of primitive division and growth during early molecular evolution. Also, the possibility that some type of encapsulation structure preceeded biochemical pathways and the assembly of genetic material is examined. These aspects will be considered from an evolutionary perspective.

cell division; molecular evolution; bacteria; self-assembly


A evolução molecular em bactérias é examinada com ênfase na auto-organização de uma célula capaz de divisão primitiva e multiplicação durante o princípio da evolução molecular. Também se discute a possibilidade de que algum tipo de estrutura de encapsulação tenha antecedido as vias bioquímicas e o agrupamento de material genético. Esses aspectos são considerados sob uma perspectiva evolutiva.

divisão celular; evolução


MOLECULAR EVOLUTION IN BACTERIA: CELL DIVISION

J.T. Trevors* * Corresponding author. Mailing address: Laboratory of Microbial Technology, Department of Environmental Biology, University of Guelph, Ontario, Canada, N1G 2W1. Fax (519) 8370442. E-mail: jtrevors@uoguelph.ca .

Laboratory of Microbial Technology, Department of Environmental Biology, University of Guelph, Guelph, Ontario, Canada

Submitted: November 03, 1998; Approved: January 18, 1999.

REVIEW

ABSTRACT

Molecular evolution in bacteria is examined with an emphasis on the self-assembly of cells capable of primitive division and growth during early molecular evolution. Also, the possibility that some type of encapsulation structure preceeded biochemical pathways and the assembly of genetic material is examined. These aspects will be considered from an evolutionary perspective.

Key words: cell division, molecular evolution, bacteria, self-assembly

INTRODUCTION Once the earliest microorganisms appeared on the Earth, the planet was changed in an irreversible manner by biological cycles, the presence of molecular oxygen from photolytic cleavage of water (27) and biological diversification of species. Since about 80% of the evolutionary history of life on Earth is restricted to microorganisms, an understanding of their molecular evolution is central to understanding the origin(s) of life and molecular evolution.

It is generally agreed that early life on Earth was capable of replicating and evolving by means of some type of primitive genetic material (4). This does not solve the mystery of the origin of the first genetic material nor how it became enclosed inside a primitive evolving, functional membrane that eventually became capable of septum formation and cell division. Moreover, evolution experimented with genetic codes and the result is diverse cellular designs, genetic coding and mechanisms of energy production and conversion that are efficient for life on Earth (18).

The physical-chemical environment in which self-assembly of primitive evolving prokaryotic cell(s) occurred was small, likely in the order of nms to µms. This makes the physical-chemical environment difficult to conceptualize when dealing with molecular evolution of bacteria. This also means parameters (e.g., temperature, pH, redox, pressure) used to describe microbial environments have no meaning on the scale of individual microbial cells (27). Evolving microbial cells would possess a large surface area to unit volume ratio needed by microorganisms that rely on nutrients of molecular dimensions (27).

The best survival mechanism in bacteria is spore formation followed by vegetative growth when the environmental conditions become favourable. If one had to suggest an early life form capable of surviving in a harsh environment, it would be bacterial spores. Indeed, the arrival of life on Earth of spores travelling through space has been suggested (7). Spores would be protected from ultraviolet light and could possibly survive for 4.5 to 45 million years in outer space. This would provide ample time to travel to Earth (26). The origin of the spores and their entry into outer space could have been due to a collision between a planet on which life existed and a meteorite (7). Another possibility is the arrival of complex organic molecules/structures on Earth from outer space. For example, complex buckyballs impacting on the Earth before being destroyed (4). This raises the possibility that other diverse, complex organic molecules also made the journey to the Earth’s surface. However, this does not elucidate how the situation on the Earth changed from a prebiotic condition to some type of genetic material such as RNA or PNA (peptide nucleic acid) and if there were numerous or only a few intermediate steps between these transitions.

There is also no absolute consensus on the types of energy sources available during early molecular evolution. Certainly in present day bacteria, the ability to obtain and store energy is central to metabolism, growth (ultimate expression of the physiology of an organism and defined as any increase in the amount of actively metabolic cytoplasm accompanied by an increase in cell numbers, cell size or both) and cell division.

Another view on the emergence of life is that it occurred about 4.2 billion years ago from iron monosulphide bubbles at a submarine hydrothermal vent (19). This type of system acted as a type of hatchery for the first organic cells. This view suggests how geochemical disequilibrium led to the first metabolizing system. It is suggested that a FeS membrane also containing nickel acted as a semipermeable catalytic boundary between the hydrothermal vent liquid and the Hadean ocean. Eventually this membrane was taken over by an organic membrane with also had a transmembrane potential. The origin of enzymes and genetic material is still unknown in this system.

Construction Kit (Elements of the Periodic Table)

During the geochemical evolution of the Earth’s chemical construction kit (e.g. periodic table of elements) the Earth would be a reducing environment. Primitive bacteria may have been autotrophs, not heterotrophs, and therefore an energy source such as pyrite derived from hydrogen sulfide and ferrous ions could be a possibility (11, 22).

Huber and Wächtershäuser (11) recently described reactions that could have been the primordial initiation reaction for the chemoautotrophic origin of life. An aqueous slurry of NiS and FeS converted CO and CH

3SH (methane-thiol) into thioacetic acid, which then hydrolyzed to acetic acid. Also, when NiS- FeS was modified with catalytic amounts of selenium; acetic acid and CH

3SH were formed from CO and H

2S. Once you have a supply of acetic acid, you may be on the path to the acetyl-CoA pathway, which is considered an ancient pathway of carbon fixation.

The results of this exciting research, support the view of a hyperthermophilic, chemoautotrophic origin of life in an iron-sulfur environment (11). The earliest organisms may have feed on carbon monoxide and/or carbon dioxide at some type of hydrothermal site.

Mineral Surfaces and Assembly of Genetic Material

Mineral surfaces would be abundant and it is not unreasonable to consider that minerals (2) and microscale molecular environments were central in early molecular evolution. A pre-RNA world may have been an unknown genetic polymer or sheet (9). It is not unreasonable to consider this genetic polymer was assembled on surfaces such as those of minerals, which would be abundant (2). Lahav and Nir (14) also recently suggested a bio-geochemical model for the emergence of template and sequence directed (TSD) synthesis. Since surfaces are virtually ubiquitous on the Earth, they may have been of central importance in early molecular evolution. However, a paucity of information and hypotheses exists on the exact role(s), if any, of surfaces on molecular evolution in the pre-DNA world.

A stable template such as a mineral surface and a means to copy the genetic material would be required. In the absence of enzyme catalysts, physical-chemical catalysts like temperature cycling, metal ions and cathode-anode gradients on surfaces (24) are interesting to speculate about. The appropriate genetic monomer units would still be required as well as the capability to make an exact or near exact copies. It is a possibility that temperature cycling may have been the physical catalyst that drove the reaction(s) in a mechanism that involved denaturation and reannealling of the growing genetic material (23). This process would act as the mechanism to glue complementary oligonucleotides or genetic material together into a stable genetic material with a high degree of constancy yet the ability to mutate or change.

The adsorption of nucleic acids to montmorillonite clay is known to be pH dependent (10) and also dependent on the length of the DNA (17). Below pH 5.0 (approximate isoelectric point for nucleic acids) adsorption of calf thymus DNA and yeast ribonucleic acid to montmorillonite is greater on internal clay surfaces. Above pH 5.0, DNA adsorption is less and restricted to external clay surfaces (10). Calcium and magnesium can cause a 2-fold increase in adsorption of nucleic acids to clays (10). The presence of KCl can cause an increase in nucleic acid adsorption and expansion of clay lattices (10). Magnesium and calcium would likely be present during early molecular evolution, and may have assisted in the binding of genetic material to mineral surfaces.

In the absence of enzyme catalysis, elongation of genetic material such as RNA may have been catalyzed by bringing monomers close to each other on a mineral surface. Temperature fluctuations may have been common. For example, geothermal hot springs, volcanic activity, deep marine thermal vents, and mineral surfaces exposed to intense sunlight during the day with cooling at night. Lower temperatures would promote adsorption and elongation/assembly of monomers while elevated temperatures would release polymers from the mineral surfaces into a water film, followed by readsorption of the growing polymeric material to clay or mineral surfaces as the temperature decreased. Repeated cycles of heating and cooling may have assisted in elongation and pairing of complementary polymers. Lahav and Nir (14) have also discussed the role of environmental cooling and heating cycles in the early template directed synthesis of emerging life. Eventually the fluctuating environment would not be the driving force and template directed synthesis involving enzymes, genetic material and the chemical construction kit contained in a semi-permeable membrane would emerge.

Joyce and Orgel (12) have summarized possible monomers for assembly of genetic material such as; hydroxy acids, amino acids, phosphomonoesters of polyhydric alcohols, aminoaldehydes and molecules containing two sulfhydryl groups. If ions such as Mg

2+ and Ca

2+ were involved, it is probable that side groups necessary to bind these elements were likely phosphate or carboxylate groups (12). Suitable compounds include aspartic acid, glutamic acid, serine phosphate, alpha-hydroxysuccinic acid and citric acid (12).

As the primitive genetic material became encapsulated (cells need an inside and outside) within a membrane, the mineral scaffold or surface could be eliminated (2). If RNA evolution preceded the formation and evolution of enzymes, genetic material may have been maintained by replication of RNA or possibly PNA (peptide nucleic acid) which was a potential prebiotic DNA precursor described by Nielson et al. (16). The PNA has a more stable backbone than DNA and can bind to itself more strongly than complementary strands of DNA (4).

Whatever the early genetic material was assembled from, it would eventually require certain characteristics. Evolving genetic material would need to be able to divide between 2 evolving cells. It would need to self-replicate, remain conserved, yet allow for some sequence diversity and have the ability to change.

Transposons and Molecular Evolution

Mobile genetic elements such as transposons in bacteria means that genome organization and evolution is a fluid dynamic process as opposed to a constant genome. Bacterial genomes are capable of relative constancy and change. Genome reorganization as a result of transposition must have been important in molecular evolution. The mobile genetic elements in addition to gene transfer mechanisms (transduction, transformation and conjugation) provides a diverse biochemical tool kit for restructuring bacterial genomes. It is however, not known if these activities were present in early molecular evolution or arrived on the scene much latter. A small mobile length of DNA such as a transposon may have been a suitable structure to assist in the assembly of evolving DNA. Transposition involves the breakage and strand transfer (mediated by the same transposase enzyme) and subsequent formation of phosphodiester bonds. As the evolving bacterial genomes became more complex and approached their optimal or present day sizes, transposition would not be as necessary. Many transposons could have been lost while some bacterial retained selected transposons which we know of today.

From an evolutionary perspective, both mobile DNA and self-catalytic RNA have unique feature that would be valuable in the self-assembly, evolution and diversification of early bacteria.

Replication and Synthesis and Early Genetic Material

It is noteworthy that replication of DNA refers to the extension of the strands of DNA. DNA synthesis refers to the actual increase in the amount of DNA regardless, of its arrangement in the chromosome. Both replication and synthesis of genetic material would also be central to the molecular evolution of early genetic material. However, both processes are dependent on the presence of enzymes and regulated biochemical pathways. Synthesis and replication of genetic material in the absence of enzymes would still require the necessary building blocks for assembly of the genetic material. A template to provide order would also be necessary as well as a means to enclose the genetic material and provide an inside and an outside to the evolving cell structure. If enzymes were present and catalytic, self-assembly of a cell would certainly be easier. The origin of a pool or library of peptides on mineral surfaces capable of catalyzing peptide bond formation is feasible (14). The selfassembly of a catalytic enzyme is likely no more difficult than self-replicating genetic material. The problem is that the self-assembly of a catalytic enzyme such as a genetic material-polymerase on a mineral surface may assist with elongation of the genetic material but it does not explain how the enzyme self-assembled multiple copies of itself. Alternatively, a primitive enzyme (or possibly one or more multifunctional enzymes with broad substrate specificity and slow catalysis capabilities) assisted with assembly of genetic material and the cell structure. A mineral surface may have acted as a support on which layers of diverse elements and molecules were brought together at the same physical micro-environment and time to allow life to commence. This type of surface assembly would increase in chemical complexity over time until some degree of order emerged due to catalytic and replicative capabilities. Increasing molecular complexity may have been very much at work in early molecular evolution. However, out of complexity, integrated metabolic pathways must emerge while obeying the laws of thermodynamics. In this case, macromolecules taking on conformations that require the least amount of free energy.

There is no explanation as to how the integration of DNA, RNA and proteins inside a cellular structure came into existence on the Earth. The construction kit (selected elements of periodic table) was likely available at specific locations or was transported by wind and water to locations. However, from the construction kit, the compounds necessary for assembly of amino acids, nucleotide bases, ribose and deoxyribose, phosphate and phospholipids for membranes had to self-assemble in a stable manner that permitted cell growth and division. Assembly is one mystery of evolution and bacterial cell division is another.

Energy and Early Evolution

If you remove organisms and oxygen from the Earth’s early atmosphere by travelling back in time about 4.9 billion years (7) you are left with mineral surfaces, water vapour and later liquid water, carbon dioxide, most likely nitrogen in the atmosphere, methane, hydrogen sulfide, sulfur dioxide and insoluble iron sulfides. Under these conditions today, bacteria such as Clostridium aceticum, capable of producing acetate from H

2 and CO

2would likely be present. Also, the sulfur metabolizing archaebacter,

Pyrodictium and

Thermoproteus are known to acquire energy by the formation of hydrogen sulfide from hydrogen and sulfur, which has been suggested to be a primeval energy source reaction (25).

The origin and subsequent evolution of the first sustainable energy-producing metabolic cycle(s) is central to an understanding of early molecular evolution. It is not unreasonable to suggest that energy producing and storing metabolic reactions may have preceded the assembly of the first genetic material. Integrated metabolic reactions are much easier to understand when enzyme catalysts are present. Enzymes regulate the pace of reactions and also regulate the sequence or order in which the reactions proceed. Otherwise the metabolic reactions necessary for life would proceed to equilibrium (none of the cells properties would change over time) and organisms would not be able to self-assemble. If the organisms were already alive they would spontaneously degrade and die. Equilibrium in living organisms can only be achieved by death. Hence, enzymes and energy would be a central requirement for early bacterial cells to grow and divide thus completing a bacterial cell cycle (period from one cell division to next cell division) (5). The first metabolic reactions would not occur with the assistance of enzymes. However, once enzymes are present, integrated biochemical pathways, replication of the genome (complete set of genetic information in a cell) and cell division all become much easier to explain. Also, some metabolic intermediates are chemically unstable and therefore it is difficult to explain their accumulation in early evolution.

Cell Division

A single bacterial cell is a complex living entity. It is difficult to hypothesize how primitive bacterial cells underwent cell division and divided cell components into two identical or virtually identical offspring cells, in most likely a chaotic environment. A functional genetic system requires protocells or cells capable of multiplying by division (6). The containment and subsequent cellularization of life allowed the cell to emerge as the basic unit of life. Moreover, the first cell had to self-assemble in an environment that supplied it needs and had a means to divide and partition genes into the offspring cells (13).

Cells need and inside and outside to divide. The fusion of peptide microspheres and simple lipids on a surface may have produced a simple spherical structure surrounding primitive genetic material. The transition from microspheres (8) (protein spheres formed by polymerization of amino acids in hot water) to a cell capable of regulated cell division requires a number of integrated molecular steps.

It is interesting to consider if a primitive microscale cathode-anode gradient separated by a primitive lipid, phospholipid or microsphere structure (Fox (8), protein spheres formed by polymerization of amino acids in hot water) provided a barrier with an electrical gradient that was a precursor of more complex membranes and eventually cell walls.

It is noteworthy that the formation of protein microspheres requires elevated temperatures (as low as 65

oC). Elevated temperatures or temperature cycling between light/dark periods would easily fulfil this temperature requirement. An anode reaction under anaerobic conditions may have been created by adsorption of hydrogen sulfide to the mineral surface while hydrogen evolution from iron sulfide surfaces could have created an anode. In addition, the cathode-anode system could have been separated by a lipid, primitive membrane barrier or microsphere on a mineral surface, that was a precursor of a more advanced membrane with a charge differential on either side of the membrane formed by the cathode-anode system (24).

Phospholipid bilayers membranes found in present day bacteria are fluid and flexible unless they undergo a transition to a gel phase (21). Encapsulation of genetic material and biochemical pathways would have had significant advantages in prokaryotic evolution. The cell as the basic unit of life could now survive by exchanges of nutrients and gases with its external environment. Encapsulation also provided the means for containing evolving protoplasm and genetic material. A self-assembling system contained by a fluid membrane would eventually be capable of division once a trigger for cell division evolved.

Phospholipid bilayers can also take on different configurations. This is a necessary feature for cells of different sizes and shapes. Moreover, phospholipid bilayers are continuous in their structure yet have the capability to self seal. They can therefore undergo fission (division) and fusion (joining together) (6) without losing their internal contents to the external environment. Phospholipid bilayers are also easy to form. Mechanical agitation can turn phospholipids into a suspension of vesicular bilayers (6). This type of self-assembling structure could then enclose the evolving biochemical pathways and genetic material. Perhaps, collisions between vesicles or microspheres brought different components into a fused structure. Or perhaps, multitudes of structures gently agitated over surfaces participated in exchanges of molecules and elements at the same physical location. However, this does not explain the origin of the phospholipids and how the necessary elements and molecules would pass into the structure to permit continued evolution. Alternatively, the first membranes could have been more rigid of a different composition, with phospholipids adding to this unknown structure to provide a more fluid, versatile membrane capable of multiple divisions. It is also important to note that membranes grow by accretion- the addition of molecules to an already existing membrane. This is a significant feature in evolution as it means that an ancestral membrane had to only self-assemble once (6). The first membrane could then enlarge and divide by fission. Division of an evolving structure may have been initially uncontrollable. For example, the structure exceeded a threshold volume and the force that holds the vesicle together was exceeded and the vesicle explodes. However, if the structure can divide before it explodes, the internal contents are preserved. The evolving cell would only need to be large enough to maintain a stable structure capable of division.

Eventually, the membrane would produce a biological potential based on a property of the membrane known as capacitance (18). Membrane capacitance is the ability to keep charges separated when there is a certain voltage (mV) transversing it. For example, a transmembrane potential can be approximately 60 mV (20). It would be difficult to explain that electrical potentials appeared in evolving cells before concentration gradients. Differences in concentrations also separate the internal cellular environment from the external surrounding environment (18). The cell can then maintain a suitable internal environment by integrated metabolism. The cell is an open system, allowing both matter and energy to pass through its boundary, the cell membrane (18). To be more accurate, the cell membrane is differentially permeable. Some substances can not leave or enter the cell.

Since membranes grow by accretion, another possibility can be considered. Suppose that self-assembling primitive membranes, vesicles, or microspheres (primitive encapsulation structure) were present before genetic material such as PNA, catalytic RNA, enzymes and any primitive evolving biochemical cycles. The membranes, vesicles or microspheres acted as the support or surface on which genetic material and the remainder of the cell self-assembled. It is not unreasonable to speculate that the primitive encapsulation structure was one of the first components of the evolving cell(s) to self-assemble as no catalysis is required for formation of the structure. Once this structure was present, genetic material and integrated biochemical cycles may have self-assembled on the inner surface and in the water-filled matrix of the structure in a somewhat protected environment. The evolving, self-assembling structure may have been more protected by the presence of clays attached to the structure or put another way, the evolving structure was attached to clay or mineral surfaces. The early presence of a contained structure could have accelerated the remaining evolution of self-assembly. The evolving pre-cell could then assemble a more complex cytoplasmic membrane capable of being energized and at the same time assembling cytoplasmic components. Genetic material could have assembled on the inner surface of the membrane, vesicle or microsphere structure. Transport of nutrients into the evolving cell(s) could have occurred via simple diffusion and gradually by the evolution and addition of facilitated diffusion and active transport. Eventually, peripheral, integral and transmembrane proteins would be added to the evolving structure. An osmotically protected evolving structure (primitive forerunner of protoplast) may have been possible. It is recognized that diffusion may not have provided all the compounds necessary for the internal assembly of the first pathways and the genetic monomers required for the assembly of genetic polymers. However, the first membranes may have had larger pores and did not exhibit a high degree of selective permeability.

DNA replication in bacterial begins at the origin of replication (ori C) which is attached to the inner surface of the cytoplasmic membrane. This relationship between DNA and the cytoplasmic membrane may have had a central role in early molecular evolution, if genetic material was ever assembled on an internal surface of a membrane. A protected internal environment of a fluid, continuous membrane is a possible surface for assembly of genetic material. The fluid nature of the structure would also permit elongating genetic material to coil. Coiling on a rigid structure would be more difficult.

Even today, some nitrifying bacteria contain internal membranes. Eukaryotic cells also require a membrane factory, the Golgi apparatus. Membranes are a central structure for cellular life forms. The earlier primitive membranes, vesicles or microspheres appear in early molecular evolution, the easier it would be for a cell to self-assemble. It is now the task to determine if some type of membrane structure with an inside and outside preceded the self-assembly of genetic material and the first biochemical pathways.

The actual mechanism of ancestral encapsulation may never be elucidated. Whatever the mechanism, the ability to proceed from an encapsulated structure to growth and division occurred. It is also possible the first cell divisions were more akin to budding (offspring cell is formed by pinching off a portion of the parent cell; a cross wall separates the bud from parent cell) and equal offspring cells from cell division evolved from a simpler budding mechanism. Actual cell division and a suitable partioning of cell contents came along latter, as a more complex regulated, process. Budding may have produced aggregates of primitive cells capable of numerous interactions at the same environmental location.

Consider the following question; what were the minimal factors/conditions involved in the self-assembly of the first primitive cell(s) that allowed it to divide by binary fission? This is not a trivial question, considering the origin of life may have occurred numerous times (15) and failed to evolve until both the molecular and environmental conditions were suitable. Bacterial cell division is central to our present understanding of microbiology as well as the origin of life forms capable of cell division. Why a bacterial cell divides is still somewhat of a mystery as complete answers are lacking (3). It has been suggested bacterial cells divide because beyond a threshold cell size, cellular life is not possible, as the rate of metabolism is insufficient for sustained life (3). The cell avoids this condition by dividing which restores a suitable cell size and surface:volume ratio necessary for normal cell processes (3). Also, when the cell size increases the surface:volume ratio decreases (3).

Another suggestion as to why cells divide is the need for genetic control. This idea states that cells require a specified amount of genome to control a specific amount of protoplasm (3). When a cell reaches a threshold size with increased amounts of protoplasm, the genome is replicated and the cell divides to restore the genome to protoplasm ratio and control is restored over the cells.

A third suggestion is that cell division occurs when sufficient biosynthetic activities have occurred to permit a new cell to exist as an individual entity (3). It is not known if the first primitive cell(s) capable of division functioned in the same manner as present day bacterial cells. One task is therefore to determine the minimal requirements for cell growth and division by the first microorganisms.

The dividing bacterial cells must also obey the rules of cell division (5). 1. Cells shall not divide unless two genomes are present. The genome has to replicate at least once. 2. Cells shall not commence DNA synthesis unless the cell has sufficient cytoplasm. The function of cell cytoplasm with respect to cell growth is to make more cytoplasm (5). This means the evolving bacterial cells had to extract energy from their environment and divide. Table 1 contains a list of suggested factors/conditions required for minimal self-assembly of a primitive cell and sufficient regulated metabolism for division.

Consider the following hypothetical situation (see Fig 1).

Figure 1. Possible evolution of biochemical pathways and genetic material in a primitive membrane, vesicle or microsphere.

1. An elevated temperature environment with 25-30 elements of the periodic table present, that are required for life as well as some simple compounds.

2. Primitive membrane, vesicle or microsphere surface(s) acting a physical, focal point for self-assembly of gentic material and biochemical pathways.

3. Early biochemical cycles for carbon-fixation and energy production and storage as the beginning of integrated metabolism.

It is not unreasonable to consider that some form of evolving biochemical pathways were needed before DNA, RNA and enzymes could be self-assembled. Perhaps these reactions occurred on the surface of a membrane, vesicle or microsphere gently bathed in a liquid, geothermal medium such as at the interface between aquatic and solid surface environments. The ability of surface metabolism to assist in the evolution of enzymes and DNA may have been the easier mechanism of cellular self-assembly from the chemical construction kit.

4. The appearance of DNA as mobile genetic material such as transposons or covalently, closed circular (CCC) DNA in the form of plasmids, that are relatively stable at an elevated temperature or under alkaline pH conditions compared to chromosomal DNA. If the early environmental conditions for evolution were harsh, plasmid DNA would be a better candidate in terms of stability. Integration of several plasmids could have lead to a small chromosome. Transposition could have brought genes together in the form of an operon or alternatively, dispersed genes.

5. Further cell membrane assembly and the ability of the cell to undergo cell division and produce 2 offspring cells capable of growth and division. Eventually cell division would lead to aggregates of cells, colonies or biofilms of bacterial cells on surfaces. Some would be dislodged and carried to new locations by flowing water or in the atmosphere as bioaerosols. Many present day bacterial species can be found as members of biofilms. In such an environment, protection from toxic compounds is afforded and transfer of genetic material can be frequent.

Archaebacteria

Archaebacteria (evolutionary distinct prokaryotes that include methanogenic, extremely halophilic and sulfur dependent bacteria, (1)) have adapted to high salt, elevated temperatures and anaerobic conditions. These are the chemical-physical conditions postulated to have been present on the early Earth. If Archaebacteria were some of the first primitive bacteria, it is important to note they have membranes that lack fatty acids, and instead contain hydrocarbon compounds bonded to glycerol by ether linkages (1). These membranes contain polar inner and outer surfaces (glycerol) and a nonpolar interior (1) that tend to spontaneously form lipid bilayers. This feature may have been necessary for primitive evolving microspheres or prokaryotic cells assembling on a mineral surface. The energized membrane is used by the cell to transport ions and uncharged molecules from the external environment into the cell (1).

Summary

It may be that future evolution in bacteria is somewhat limited. Bacteria participate in gene transfer events such as transformation, transduction and conjugation and are also subjected to mutation events. However, it is possible that the single chromosome in different species of bacteria has reached a near maximum size and future bacterial evolution is restricted to fewer changes. For example, metabolic pathways for chemical pollutants in the environment and increasing numbers and species of multiple antibiotic resistant bacteria or emerging plant and animal pathogen microorganisms. Of course, some of these ideas are only speculation.

If any part of the bacterial cell such as DNA, RNA or proteins is removed, the ability of bacterial cells to grow, divide and remain viable in the environment is impossible. Therefore, it is reasonable to suggest that the first cell(s) capable of a division and subsequent growth and additional divisions, must have been relatively advanced in their structure and function. This first cell capable of binary division was likely the universal ancestor from which life evolved and diversified.

As more research is forthcoming on self-assembly and molecular evolution in bacteria, the scientific community may gain new insights into the origin of prokaryotic cell division. It is also possible that the theoretical ideas discussed in this manuscript may be partially or completely incorrect. An integration of scientific information from the physical, chemical and biological sciences and additional research on the early self-assembly of life and how cells divide is needed to stimulate new research and debates on molecular evolution.

ACKNOWLEDGMENTS Appreciation is expressed to the Natural Sciences and Engineering Research Council of Canada (NSERC) operating grants program for financial support of research in my laboratory.

RESUMO

Evolução molecular em bactérias: divisão celular

A evolução molecular em bactérias é examinada com ênfase na auto-organização de uma célula capaz de divisão primitiva e multiplicação durante o princípio da evolução molecular. Também se discute a possibilidade de que algum tipo de estrutura de encapsulação tenha antecedido as vias bioquímicas e o agrupamento de material genético. Esses aspectos são considerados sob uma perspectiva evolutiva.

Palavras-chave: divisão celular, evolução

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*
Corresponding author. Mailing address: Laboratory of Microbial Technology, Department of Environmental Biology, University of Guelph, Ontario, Canada, N1G 2W1. Fax (519) 8370442. E-mail:

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Publication Dates

  • Publication in this collection
    27 May 1999
  • Date of issue
    Oct 1998

History

  • Received
    03 Nov 1998
  • Accepted
    18 Jan 1999
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