Figure 1 - The general process of mRNA synthesis in trypanosomatids. Most trypanosome and Leishmania genes characterized thus far are organized in tandem repeats as indicated by the shaded boxes (representing two copies of an A gene and one copy of a B gene). At another chromosomal location there are several hundred tandem direct repeats encoding SL RNAs (dark boxes). In some cases, a single promoter is present upstream of the first gene in the cluster whereas in the SL gene cluster each repeat is presumed to bear a transcriptional promoter (small flags). After transcription, the polycistronic pre-mRNA is processed by trans-splicing and polyadenylation, which require signals present within the intergenic regions: a polypyrimidine-rich sequence or (CT)n (small boxes), the spliced leader addition site or AG (arrowheads), and a polyadenylation addition site or poly(A) (arrows). Mature mRNAs which consist of the coding region and 5'UTR (shaded boxes), the SL sequence (dark boxes), 3'UTR and poly(A) tail (thin lines with small "A" letters) are represented by the diagram at the bottom of the figure.