PERSISTENCE OF AN ALTERNATE CHROMATIN STRUCTURE AT SILENCED LOCI IN THE ABSENCE OF SILENCERS

In Saccharomyces cerevisiae, genes placed near telomeres or the silent HML and HMR mating-type loci are transcriptionally repressed by a het erochromatin-like structure. We have generated nonreplicating DNA ring s by recombination in vivo to examine the role of chromosomal context on transcriptional repression. Specifically, recombination at HMR was used to produce rings that lacked the E and I silencers. An altered le vel of DNA supercoiling was observed in these rings but not in compara ble rings from derepressed loci. Our results indicate that a repressiv e chromatin structure persists in an extrachromosomal environment imme diately following removal of the cis-acting control elements. Examinat ion of both chromatin footprints and DNA sequence dependence revealed that changes in nucleosome number could account for the topology shift s. Upon continued cell growth, the differences in supercoiling were lo st and transcriptional competence was restored. These results show tha t silencers are required for sustained persistence of repressive chrom atin structure, even in the absence of DNA replication.