The yeast Ty5 retrotransposon preferentially integrates into heterochromatin at the telomeres and silent mating loci. Target specificity is mediated by a small domain of Ty5 integrase (the targeting domain, TD), which interacts with the heterochromatin protein Sir4 and tethers the integration complex to target sites. Here we demonstrate that TD is phosphorylated and that phosphorylation is required for interaction with Sir4. The yeast cell, therefore, through posttranslational modification, controls Ty5's mutagenic potential: when TD is phosphorylated, insertions occur in gene-poor heterochromatin, thereby minimizing deleterious consequences of transposition; however, in the absence of phosphorylation, Ty5 integrates throughout the genome, frequently causing mutations. TD phosphorylation is reduced under stress conditions, specifically starvation for amino acids, nitrogen, or fermentable carbon. This suggests that Ty5 target specificity changes in response to nutrient availability and is consistent with McClintock's hypothesis that mobile elements restructure host genomes as an adaptive response to environmental challenge.
Bibliographical noteFunding Information:
This work was supported by National Institutes of Health Grant GM061657. We thank William Lewis and Siquan Luo at the Iowa State University Proteomics Facility for carrying out the MS and MS/MS analysis. None of the authors has a conflict of interest related to the work reported in this manuscript.