YEAST RADS PROTEIN BINDS DIRECTLY TO BOTH SSL2 AND SSL1 PROTEINS - IMPLICATIONS FOR THE STRUCTURE AND FUNCTION OF TRANSCRIPTION REPAIR FACTOR-B/

The RAD3 and SSL2 gene products are essential proteins that are also r equired for the nucleotide excision repair pathway. We have recently d emonstrated that the RAD3 gene product along with the SSL1 and TFB1 ge ne products are subunits of RNA polymerase II basal transcription fact or b. Additionally, the SSL2 gene product physically interacts with pu rified factor b. Here we combine an in vitro immunoprecipitation assay and an in vivo genetic assay to demonstrate a series of pairwise prot ein-protein interactions involving these components. RAD3 protein bind s directly to both SSL2 protein and SSL1 protein in vitro. SSL1 protei n interacts with itself and with RAD3 and TFB1 proteins in living yeas t cells. An N-terminal, possibly noncatalytic, domain of SSL2 protein is sufficient for the factor b-SSL2 interaction, and a product of a DN A repair-defective allele of SSL2 is not defective in binding to facto r b. We present genetic evidence suggesting that the DNA-repair functi on of SSL2 protein is not dependent on its essential function.