SIN1 interacts with a protein that binds the URS1 region of the yeast HO gene

Evidence has recently been mounting suggesting that a number of chromatin components previously thought to primarily or exclusively have structural function, also have a regulatory role in eukaryotic transcription. Notably, in yeast, histone H4 N-terminal sequence has been shown to be required for promoter activation of certain genes in vivo, and mutations in histone H3 (SIN2) or in SIN1 (which has some sequence similarity to HMG1) are able to suppress swi1, swi2, and swi3 mutations, restoring transcription to HO as well as a number of other genes. In this paper we report the identification of a novel protein or protein complex that specifically binds a short sequence in the HO regulatory region 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 sin1 delta strains retain the activity. Interestingly, extracts made from cells carrying the dominant sin1-2 point mutation lack the binding activity. Furthermore, bacterially produced sin1-2 protein can dissociate a DNA/protein complex while a similarly produced SIN1 protein has no effect on the complex at similar 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 data imply that a sequence specific DNA binding protein(s) may mediate between the SIN1 protein and the basal transcription apparatus transcribing HO.