Nucleosomes are positioned with base pair precision adjacent to the alpha 2 operator in Saccharomyces cerevisiae

Analysis of the chromatin structure of minichromosomes containing the binding site for the yeast alpha 2 repressor protein by indirect end-labeling has previously indicated that nucleosomes are stably positioned over sequences adjacent to the alpha 2 operator in the presence of the repressor. Development of a primer extension assay for nucleosome position now allows a more detailed examination of the location of these nucleosomes relative to the operator sequence, and indicates that nucleosomes are precisely and stably positioned both translationally and rotationally over sequences adjoining the operator. In addition, this assay enables analysis of the chromatin structure of single copy, genomic sequences. Chromatin structures determined for two genes regulated by alpha 2, STE6 and BAR1, are consistent with nucleosomes precisely positioned downstream of the operator sequence, incorporating promoter elements, in alpha cells but not in a-cells. The location of these nucleosomes relative to the operator sequence is highly analogous to that observed in the minichromosome. The stability of the nucleosomes adjacent to the operator together with the precision of their location suggests that they may play a role in repression of a specific gene expression by alpha 2. Further, the primer extension assay allows a comparison of the structure of these positioned nucleosomes formed in vivo to that previously described for core particles reconstituted in vitro.