Stimulation of mitotic recombination events by high levels of RNA polymerase II transcription in yeast

The impact of high levels of RNA polymerase II transcription on mitotic rec ornbination was examined using lys2 recombination substrates positioned on nonhomologous chromosomes. Substrates were used that could produce Lys(+) r ecombinants by either a simple (noncrossover) gene conversion event or a cr ossover-associated recombination event, by only a simple gene conversion ev ent, or by only a crossover event. Transcription of the lys2 substrates was regulated by the highly inducible GAL1-10 promoter or the low-level LYS2 p romoter, with GAL1-10 promoter activity being controlled by the presence or absence of the Gal80p negative regulatory protein. Transcription was found to stimulate recombination in all assays used, but the level of stimulatio n varied depending on whether only one or both substrates were highly trans cribed. In addition, there was an asymmetry in the types of recombination e vents observed when one substrate versus the other was highly transcribed. Finally, the lys2 substrates were positioned as direct repeats on the same chromosome and were found to exhibit a different recombinational response t o high levels of transcription from that exhibited by the repeats on nonhom ologous chromosomes. The relevance of these results to the mechanisms of tr anscription-associated recombination are discussed.