THE SACCHAROMYCES RETROTRANSPOSON TY5 INTEGRATES PREFERENTIALLY INTO REGIONS OF SILENT CHROMATIN AT THE TELOMERES AND MATING LOCI

The nonrandom integration of retrotransposons and retroviruses suggest s that chromatin influences target choice. Targeted integration, in tu rn, likely affects genome organization. In Saccharomyces, native Ty5 r etrotransposons are located near telomeres and the silent mating locus HMR. To determine whether this distribution is a consequence of targe ted integration, we isolated a transposition-competent Ty5 element fro m S. paradoxus, a species closely related to S. cerevisiae. This Ty5 e lement was used to develop a transposition assay in S. cerevisiae to i nvestigate target preference of de novo transposition events. Of 87 in dependent Ty5 insertions, similar to 30% were located on chromosome II I, indicating this small chromosome (similar to 1/40 of the yeast geno me) is a highly preferred target. Mapping of the exact location of 19 chromosome III insertions showed that 18 were within or adjacent to tr anscriptional silencers flanking HML and HMR or the type X subtelomeri c repeat. We predict Ty5 target preference is attributable to interact ions between transposition intermediates and constituents of silent ch romatin assembled at these sites. Ty5 target preference extends the li nk between telomere structure and reverse transcription as carried out by telomerase and Drosophila retrotransposons.