THE SACCHAROMYCES-CEREVISIAE SPT8 GENE ENCODES A VERY ACIDIC PROTEIN THAT IS FUNCTIONALLY RELATED TO SPT3 AND TATA-BINDING PROTEIN

Mutations in the Saccharomyces cerevisiae SPT8 gene were previously is olated as suppressors of retrotransposon insertion mutations in the 5' regions of the HIS4 and LYS2 genes. Mutations in SPT8 confer phenotyp es similar to those caused by particular mutations in SPT15, which enc odes the TATA-binding protein (TBP). These phenotypes are also similar to those caused by mutations in the SPT3 gene, which encodes a protei n that directly interacts with TBP. We have now cloned and sequenced t he SPT8 gene and have shown that it encodes a predicted protein of 602 amino acids with an extremely acidic amino terminus. In addition, the predicted SPT8 amino acid sequence contains one copy of a sequence mo tif found in multiple copies in a number of other eukaryotic proteins, including the P subunit of heterotrimeric G proteins. To investigate further the relationship between SPT8, SPT3 and TBP, we have analyzed the effect of an spt8 null mutation in combination with different spt3 and spt15 mutations. This genetic analysis has shown that an spt8 del etion mutation is suppressed by particular spt3 alleles. Taken togethe r with previous results, these data suggest that the SPT8 protein is r equired, directly or indirectly, for TBP function at particular promot ers and that the role of SPT8 may be to promote a functional interacti on between SPT3 and TBP.