Transcriptional effects on double-strand break-induced gene conversion tracts

Transcription stimulates spontaneous homologous recombination, but prior st udies have not investigated the effects of transcription on double-strand b reak (DSB)-induced recombination in yeast. We examined products of five ura 3 direct repeat substrates in yeast using alleles that were transcribed at low or high levels. In each strain, recombination was stimulated by DSBs cr eated in vivo at an HO site in one copy of ura3. Increasing transcription l evels in donor or recipient alleles did not further stimulate DSB-induced r ecombination, nor did it alter the relative frequencies of conversion and d eletion (pop-out) events. This result is consistent with the idea that tran scription enhances spontaneous recombination by increasing initiation. Gene conversion tracts were measured using silent restriction fragment length p olymorphisms (RFLPs) at approximately 100 bp intervals. Transcription did n ot alter average tract lengths, but increased transcription in donor allele s increased both the frequency of promoter-proximal (5') unidirectional tra cts and conversion of 5' markers. Increased transcription in recipient alle les increased the frequency of bidirectional tracts. We demonstrate that th ese effects are due to transcription per se, and not just transcription fac tor binding. These results suggest that transcription influences aspects of gene conversion after initiation, such as strand invasion and/or mismatch repair (MMR). (C) 2000 Elsevier Science B.V. All rights reserved.