STIMULATION OF MITOTIC RECOMBINATION UPON TRANSCRIPTION FROM THE YEAST GAL1 PROMOTER BUT NOT FROM OTHER RNA-POLYMERASE-I, RNA-POLYMERASE-IIAND RNA-POLYMERASE-III PROMOTERS

Homologous recombination in Saccharomyces cerevisiae and other organis ms can be stimulated by transcription. Consistent with this, we find t hat recombination of a chromosomal ade1 allele with a plasmid-borne AD E1 ORF under the control of the GAL1 promoter increased from 6.1 x 10( -6) to 1.7 x 10(-4) when transcription of the plasmid locus was induce d by growing the cells in the presence of galactose. Recombination cou ld also be stimulated by over-expressing the Gal4 transcription factor in the presence of the GAL1-ADE1 plasmid, while culturing the cells i n dextrose medium. However, when transcription of the same ORF was dri ven from the highly active promoters of the rDNA (RNA polymerase I), a nd ADH1 (RNA polymerase II) genes, only background levels of recombina tion (5-10 x 10(-6)) were observed, irrespective of the carbon source. Recombination was found to involve integration of the whole plasmid a nd to depend on RAD51, RAD52 and RAD54. The results indicate that incr eased accessibility of transcriptionally active chromatin is not suffi cient to cause increased rates of this kind of reciprocal exchange.