Characterization of subunits of the RNA polymerase I complex in Trypanosoma brucei

The Trypanosoma brucei homologue of the RNA polymerase I (RNA Pol I) subunit Rpa12p of Saccharomyces cerevisiae was cloned and characterized. This protein did not appear to be essential for growth in either bloodstream or procyclic forms of the parasite. Trypanosomes expressing a C-terminal tagged version of TbRPA12 were generated in order to purify RNA Pol I from both developmental stages. Tandem affinity purification (TAP) revealed a number of proteins associating with TbRPA12, some of which appeared to be stage-specific. Mass spectrometry allowed the identification of four subunits in addition to TbRPA12, namely TbRPA1, TbRPA2, TbRPC40 and one isoform of TbRPB5 (Tb1RPB5), as well as an unknown 30 kDa protein and histones H2A and H3. Whereas these studies demonstrated that TbRPA1 was phosphorylated, no evidence for phosphorylation of TbRPA2 was found.