Host regulation of the replication of the yeast retrotransposon Ty5
The replication of retroelements may have deleterious effects on the hosts. Therefore, elements and their hosts have coevolved mechanisms to regulate retroelement replication. This regulation can occur at the transcriptional, translational, or post-translational levels as well as through targeted integration or cDNA recombination. I have studied host regulation of the Saccharomyces retrotransposon Ty5. I have found that Ty5 transcription is carefully controlled. Transcription of Ty5 is haploid specific and is repressed 10 fold in diploid strains. Ty5 transcription is also regulated by the pheromone response pathway, which is activated during mating. In addition, Ty5 transcription is regulated by flanking sequences. Ty5 preferentially integrates into regions of silent chromatin at the telomeres and the HM loci, which, in turn, represses Ty5 transcription. Although the repressive effects of silent chromatin have been well characterized, I have found that transcription and transposition of Ty5 elements embedded in silent chromatin can be induced by pheromone treatment or during the mating process. Another level of retroelement regulation is the targeting of element cDNA to non-coding regions. cDNA can be targeted by the element-encoded integrase protein or through recombination with pre-existing elements. I found that Ty5 recombines at high frequency with homologous substrates. Two classes of recombinants were identified: tandem elements and elements that arose by marker exchange between the Ty5 cDNA and the substrate. Ty5 cDNA recombination is dependent on the host repair protein Rad52, but not on the host repair protein Rad1 or Ty5 integrase. Using an assay system that allowed me to identify the cis-acting sequences important Ty5 cDNA recombination, I have found that the Ty5 LTR is critical for tandem element formation. Internal Ty5 sequences can facilitate tandem element formation in the presence of an LTR. These data have made it possible for me to propose models for tandem element formation based on recombination between the LTRs of the cDNA and substrate. The LTR was not found to be important for marker exchange events, suggesting that the two classes of recombinants arise by different mechanisms. Targeted integration and recombinational amplification of Ty5 elements at the telomeres suggests that Ty5 may play an important role in telomere structure.