THE PHEROMONE RESPONSE PATHWAY ACTIVATES TRANSCRIPTION OF TY5 RETROTRANSPOSONS LOCATED WITHIN SILENT CHROMATIN OF SACCHAROMYCES-CEREVISIAE

The Saccharomyces retrotransposon Ty5 integrates preferentially into t ranscriptionally inactive regions (silent chromatin) at the HM loci an d telomeres. We found that silent chromatin represses basal Ty5 transc ription, indicating that these elements are encompassed by silent chro matin in their native genomic context. Because transcription is a requ irement for transposition, integration into silent chromatin mould app ear to prevent subsequent rounds of replication, Using plasmid-borne T y5-lacZ constructs, we found that Ty5 expression is haploid specific a nd is repressed 10-fold in diploid strains. Ty5 transcription is also regulated by the pheromone response pathway and is induced similar to 20-fold upon pheromone treatment. Deletion analysis of the Ty5 LTR pro moter revealed that a 33 bp region with three perfect matches to the p heromone response element is responsible for both mating pheromone and cell-type regulation. Transcriptional repression of Ty5 by silent chr omatin can be reversed by pheromone treatment, which leads to transcri ption and transposition. Ty5 replication, therefore, is normally repre ssed by silent chromatin and appears to be induced during mating, This is the first example of transcriptional activation of a gene that nat urally resides within silent chromatin.