Domains of Gene Silencing Near the Left End of Chromosome III in Saccharomyces cerevisiae

In Saccharomyces cerevisiae the HM loci and regions adjacent to the telomeres are transcriptionally silent. HML is situated 11 kb from the left telomere of chromosome III. I have systematically examined gene silencing along this 11-kb chromosomal region. I found that silencing extends at least 1.1 kb beyond HML, indicating that the HML E silencer acts on both sides. Moreover, I obtained evidence indicating that a 0.71-kb sequence near the E silencer acts as a barrier to the spread of silencing and coincides with the left boundary of the silent HML domain. I also showed that silencing at the telomere is limited to an approximately 2-kb domain. On the other hand, an approximately 7-kb region between HML and the telomere is not silenced by HML or the telomere. These results provide a clear example of organization of the eukaryotic genome into interspersed domains with distinct potentials for gene expression.