DNA-protein interactions at the S.cerevisiae α2 operator in vivo

Two homodlmerlc proteins, α2 and MCM1, are required to repress transcription of a-cell type specific genes In haplold yeast α-cells. In vitro studies by others of the Interactions of these proteins with operator DNA have suggested that MCM1 binds to the middle and α2 to the ends of the 31 bp operator. We have previously shown that α2 organizes chromatin structure adjacent to the operator; In the presence of α2 repressor, a precisely positioned nucleosome abuts the operator in both minichromosomes and the genome. We present In vitro footprintlng evidence consistent with occupancy of the operator by MCM1 In both a- and α-cells and by α2 repressor in α-cells. Interestingly, our In vitro results differ from previous In vitro work in detail. In contrast to the broad block of reagent accessibility to DNA by the factors seen In vitro, we find a pattern of strand-specific protection or augmented reactivity In vitro. The In vivo results are consistent with genetic data concerning transcriptlonal regulation of a-cell specific genes and corroborate the crystallographic data of others.