An in silico analysis of trypanosomatid RNA polymerases: insights into their unusual transcription.

African trypanosomes employ both Pol I (RNA polymerase I) and Pol II to transcribe protein-coding genes in large polycistronic units of up to 50 genes. Subsequent processing produces mature capped mRNAs. Evidence suggests that regulation of gene expression is primarily exerted post-transcriptionally. Here, we use the recently completed genome sequences of three trypanosomatids, Trypanosoma brucei, Trypanosoma cruzi and Leishmania major, in an in silico analysis of their fundamental RNA polymerase complexes. The core complement of Pol II subunits, including those that are shared with Pol I and Pol III are present. However, both Pol I and Pol III complexes are missing members of the rpoE-rpoF subunit groups. Out of the five shared subunits, both RPB5 and RPB6 have two isoforms in the three trypanosomes. One represents the canonical polymerase subunit and the other differs by insertion or deletion of stretches of charged residues. We propose that these alternative isoforms function in distinct polymerase complexes, and may influence recruitment of the trypanosome RPB4-RPB7 heterodimer.