THE SACCHAROMYCES TY5 RETROTRANSPOSON FAMILY IS ASSOCIATED WITH ORIGINS OF DNA-REPLICATION AT THE TELOMERES AND THE SILENT MATING LOCUS HMR

We have characterized the genomic organization of the Ty5 retrotranspo sons among diverse strains of Saccharomyces cerevisiae and the related species Saccharomyces paradoxus. The S. cerevisiae strain S288C (or i ts derivatives) carries eight Ty5 insertions. Six of these are located near the telomeres, and five are found within 500 bp of autonomously replicating sequences present in the type X subtelomeric repeat. The r emaining two S. cerevisiae elements are adjacent to the silent mating Locus HMR and are located within 500 bp of the origin of replication p resent in the transcriptional silencer HMR-E. Although the S. cerevisi ae Ty5 elements no longer appear capable of transposition, some strain s of S. paradoxus have numerous Ty5 insertions, suggesting that transp osition is occurring in this species. Most of these elements are adjac ent to type X telomeric repeats, and regions flanking four of five cha racterized S. paradoxus insertions carry autonomously replicating sequ ences. The genomic organization of the Ty5 elements is in marked contr ast to the other S. cerevisiae retrotransposon families (Ty1-4), which are typically located within 500 bp of tRNA genes. For Ty3, this asso ciation reflects an interaction between Ty3 and the RNA polymerase III transcription complex, which appears to direct integration [Chalker, D. L. and Sandmeyer, S. B. (1992) Genes Dev. 6, 117-128]. By analogy t o Ty3, we predict that Ty5 target choice is specified by interactions with factors present at both the telomeres and HMR that are involved i n DNA replication, transcriptional silencing, or the maintenance of th e unique chromatin structure at these sites.