DNA-PROTEIN INTERACTIONS AT THE SACCHAROMYCES-CEREVISIAE ALPHA-2-OPERATOR IN-VIVO

Two homodimeric proteins, alpha2 and MCM1, are required to repress tra nscription of a-cell type specific genes in haploid yeast alpha-cells. In vitro studies by others of the interactions of these proteins with operator DNA have suggested that MCM1 binds to the middle and alpha2 to the ends of the 31 bp operator. We have previously shown that alpha 2 organizes chromatin structure adjacent to the operator; in the prese nce of alpha2 repressor, a precisely positioned nucleosome abuts the o perator in both minichromosomes and the genome. We present in vivo foo tprinting evidence consistent with occupancy of the operator by MCM1 i n both a- and alpha-cells and by alpha2 repressor in alpha-cells. Inte restingly, our in vivo results differ from previous in vitro work in d etail. In contrast to the broad block of reagent accessibility to DNA by the factors seen in vitro, we find a pattern of strand-specific pro tection or augmented reactivity in vivo. The in vivo results are consi stent with genetic data concerning transcriptional regulation of a-cel l specific genes and corroborate the crystallographic data of others.