DNA- protein interactions attheS.cerevisiae au2operator invivo

Twohomodimeric proteins, a2andMCM1,arerequired torepress transcription ofa-cell typespecific genesin haploid yeasta-cells. Invitro studies byothers ofthe interactions ofthese proteins withoperator DNAhave suggested that MCM1binds tothemiddle anda2tothe endsofthe31bpoperator. Wehavepreviously shown that a2organizes chromatin structure adjacent tothe operator; inthepresence ofa2repressor, aprecisely positioned nucleosome abutstheoperator inboth minichromosomes andthegenome. Wepresent invivo footprinting evidence consistent withoccupancy ofthe operator byMCM1inbotha-anda-cells andbya2 repressor ina-cells. Interestingly, ourinvivoresults differ fromprevious invitro workindetail. Incontrast tothebroadblock ofreagent accessibility toDNAby thefactors seeninvitro, wefind apattern ofstrandspecific protection oraugmented reactivity invivo. The invivoresults areconsistent withgenetic data concerning transcriptional regulation ofa-cell specific genesandcorroborate thecrystallographic dataof others.