Transcriptional induction of Ty recombination in yeast.

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Abstract
Families of repeated sequences are present in the genomes of all eukaryotes. Little is known about the mechanism(s) that prevents recombination between repeated sequences. In the yeast Saccharomyces cerevisiae, recombination between homologous sequences placed at nonhomologous locations in the genome (ectopic recombination) has been shown to occur at high frequencies for artificially created repeats, but at relatively low frequencies for a natural family of repeated sequences, the Ty family. We have previously shown that a high level of Ty cDNA in the cell causes an increase in the rate of nonreciprocal recombination (gene conversion) of a marked Ty element. In the present study, we show that it is also possible to elevate the rate of recombination of a marked Ty by increasing its transcription. This induction is different from, and acts synergistically to, the one seen upon increased levels of donor Ty cDNA. We show that the induction by transcription does not require the products of the RAD50, RAD51, and RAD57 genes. In contrast, cDNA-mediated recombination is dependent on the product of the RAD51 gene but not on products of the genes RAD50 or RAD57.