SIN1 INTERACTS WITH A PROTEIN THAT BINDS THE URS1 REGION OF THE YEASTHO GENE

Evidence has recently been mounting suggesting that a number of chroma tin components previously thought to primarily or exclusively have str uctural function, also have a regulatory role in eukaryotic transcript ion. Notably, in yeast, histone H4 N-terminal sequence has been shown to be required for promoter activation of certain genes in vivo, and m utations in histone H3 (SIN2) or in SIN1 (which has some sequence simi larity to HMG1) are able to suppress swi1, swi2, and swi3 mutations, r estoring transcription to HO as well as a number of other genes. In th is paper we report the identification of a novel protein or protein co mplex that specifically binds a short sequence in the HO regulatory re gion on the one hand, and on the other somehow appears to contact the SIN1 protein. We have shown that the DNA binding activity itself does not contain SIN1, since extracts from sin1DELTA strains retain the act ivity. Interestingly, extracts made from cells carrying the dominant s in1-2 point mutation lack the binding activity. Furthermore, bacterial ly produced sin1-2 protein can dissociate a DNA/protein complex while a similarly produced SIN1 protein has no effect on the complex at simi lar concentrations. When the DNA sequence to which the protein complex binds is placed in a CYC1 promoter lacking a UAS (upstream activating sequence), it can serve as a weak UAS in a SIN1 dependent way. Our da ta imply that a sequence specific DNA binding protein(s) may mediate b etween the SIN1 protein and the basal transcription apparatus transcri bing HO.