FUNCTIONAL INTERACTION BETWEEN TFIIB AND THE RPB9 (SSU73) SUBUNIT OF RNA-POLYMERASE-II IN SACCHAROMYCES-CEREVISIAE

Recessive mutations in the SSU71, SSU72 and SSU73 genes of Saccharomyc es cerevisiae were identified as either suppressors or enhancers of a TFIIB defect (sua7-1) that confers both a cold-sensitive growth phenot ype and a downstream shift in transcription start site selection, The SSU71 (TFG1) gene encodes the largest subunit of TFIIF and SSU72 encod es a novel protein that is essential for cell viability, Here we repor t that SSU73 is identical to RPB9, the gene encoding the 14.2 kDa subu nit of RNA polymerase II, The ssu73-1 suppressor compensates for both the growth defect and the downstream shift in start site selection ass ociated with sua7-1, These effects are similar to those of the ssu71-1 suppressor and distinct from the ssu72-1 enhancer The ssu73-1 allele was retrieved and sequenced, revealing a nonsense mutation at codon 10 7, Consequently, ssu73-1 encodes a truncated form of Rpb9 lacking the C-terminal 16 amino acids, This Rpb9 derivative retains at least parti al function since the ssu73-1 mutant exhibits none of the growth defec ts associated with rpb9 null mutants. However, in a SUA7(+) background , ssu73-1 confers the same upstream shift at ADH1 as an rpb9 null alle le, This suggests that the C-terminus of Rpb9 functions in start site selection and demonstrates that the previously observed effects of rpb 9 mutations on start site selection are not necessarily due to complet e loss of function, These results establish a functional interaction b etween TFIIB and the Rpb9 subunit of RNA polymerase II and suggest tha t these two components of the preinitiation complex interact during tr anscription start site selection.