A FUNCTIONAL-ROLE FOR NUCLEOSOMES IN THE REPRESSION OF A YEAST PROMOTER

Induction of the PH05 gene in S. cerevisiae was previously shown to be accompanied by the removal of four positioned nucleosomes from the promoter. In order to assess the role of nucleosomes in the cascade of gene activation, DNA corresponding to one of these nucleosomes was excised. In its place two foreign DNA segments of the same length were inserted: a fragment from the African green monkey alpha-satellite DNA which is known to associate with histones in a highly specific fashion to give a uniquely positioned nucleosome or alternatively, a fragment derived from pBR322 DNA. The promoter constructs were fused to the lacZ gene on centromere plasmids and transformed into yeast cells. The satellite fragment formed a nucleosome which persisted under inducing conditions. At the same time the inducibility of the PH05 promoter was virtually abolished. When various subfragments containing between 35 and 100 bp of the satellite segment were tested, they were all found to decrease the inducibility of the promoter, full repression required the full length molecule, however. In contrast, the pBR fragment made the promoter weakly constitutive, and induction proceeded to levels even higher than with a promoter lacking an insert. Analysis of the chromatin structure reveals a nucleosome on the pBR segment at noninducing conditions which is removed upon induction. It is concluded that the quality of the histone - DNA interactions at the promoter makes an intrinsic contribution to the regulation of the gene.