Transcription by the multifunctional RNA polymerase I in Trypanosoma brucei functions independently of RPB7.

Trypanosoma brucei has a multifunctional RNA polymerase (pol) I that transcribes ribosomal gene units (RRNA) and units encoding its major cell surface proteins variant surface glycoprotein (VSG) and procyclin. Previous analysis of tandem affinity-purified, transcriptionally active RNA pol I identified ten subunits including an apparently trypanosomatid-specific protein termed RPA31. Another ortholog was identified in silico. No orthologs of the yeast subunit doublet RPA43/RPA14 have been identified yet. Instead, a recent report presented evidence that RPB7, the RNA pol II paralog of RPA43, is an RNA pol I subunit and essential for RRNA and VSG transcription in bloodstream form trypanosomes [18]. Revisiting this attractive hypothesis, we were unable to detect a stable interaction between RPB7 and RNA pol I in either reciprocal co-immunoprecipitation or tandem affinity purification. Furthermore, immunodepletion of RPB7 from extract virtually abolished RNA pol II transcription in vitro but had no effect on RRNA or VSG ES promoter transcription in the same reactions. Accordingly, chromatin immunoprecipitation analysis revealed cross-linking of RPB7 to known RNA pol II transcription units but not to the VSG ES promoter or to the 18S rRNA coding region. Interestingly, RPB7 did crosslink to the RRNA promoter but so did the RNA pol II-specific subunit RPB9 suggesting that RNA pol II is recruited to this promoter. Overall, our data led to the conclusion that RNA pol I transcription in T. brucei does not require the RNA pol II subunit RPB7.