Structure of the C-terminal domain of transcription factor IIB from Trypanosoma brucei.

In trypanosomes, the production of mRNA relies on the synthesis of the spliced leader (SL) RNA. Expression of the SL RNA is initiated at the only known RNA polymerase II promoter in these parasites. In the pathogenic trypanosome, Trypanosoma brucei, transcription factor IIB (tTFIIB) is essential for SL RNA gene transcription and cell viability, but has a highly divergent primary sequence in comparison to TFIIB in well-studied eukaryotes. Here we describe the 2.3 A resolution structure of the C-terminal domain of tTFIIB (tTFIIB(C)). The tTFIIB(C) structure consists of 2 closely packed helical modules followed by a C-terminal extension of 32 aa. Using the structure as a guide, alanine substitutions of basic residues in regions analogous to functionally important regions of the well-studied eukaryotic TFIIB support conservation of a general mechanism of TFIIB function in eukaryotes. Strikingly, tTFIIB(C) contains additional loops and helices, and, in contrast to the highly basic DNA binding surface of human TFIIB, contains a neutral surface in the corresponding region. These attributes probably mediate trypanosome-specific interactions and have implications for the apparent bidirectional transcription by RNA polymerase II in protein-encoding gene expression in these organisms.