Characterization of the RNA polymerases of Trypanosoma brucei: trypanosomal mRNAs are composed of transcripts derived from both RNA polymerase II and III.

To analyze transcription in Typanosoma brucei, we have characterized the trypanosomal RNA polymerases. Here we present our results, which allow a discrimination between the different classes of RNA polymerases in nuclear run-on experiments by polymerase inhibitors and Mn2+ dependence. We also describe the separation of trypanosomal RNA polymerases by chromatography, demonstrating that T. brucei contains RNA polymerases I-III. The outcome of our experiments suggests that the VSG genes of T. brucei are not transcribed by RNA polymerase I, as previously reported, but by RNA polymerase II. We propose that an additional factor modifies RNA polymerase II, resulting in the alpha-amanitin-resistant transcription of VSG genes. Our data also suggest that the mini-exon genes, which encode the 5' end of each trypanosomal mRNA, are probably transcribed by RNA polymerase III.