High-Resolution Mapping of Two Types of Spontaneous Mitotic Gene Conversion Events in Saccharomyces cerevisiae
Eunice Yim, Karen E. O'Connell, Jordan St. Charles, Thomas D. Petes

Abstract

Gene conversions and crossovers are related products of the repair of double-stranded DNA breaks by homologous recombination. Most previous studies of mitotic gene conversion events have been restricted to measuring conversion tracts that are less than 5 kb. Using a genetic assay in which the lengths of very long gene conversion tracts can be measured, we detected two types of conversions, those that have a median size of about 6 kb and those with a median size of more than 50 kb. The unusually long tracts are initiated at a naturally-occurring recombination hotspot formed by two inverted Ty elements. We suggest that these long gene conversion events may be generated by a different mechanism (break-induced replication or repair of a double-stranded DNA gap) than the short conversion tracts that likely reflect heteroduplex formation followed by DNA mismatch repair. Both the short and long mitotic conversion tracts are considerably longer than those observed in meiosis. Since mitotic crossovers in a diploid can result in a heterozygous recessive deleterious mutation becoming homozygous, it has been suggested that the repair of DNA breaks by mitotic recombination involves gene conversion events that are unassociated with crossing over. In contrast to this prediction, we found that about 40% of the conversion tracts are associated with crossovers. Spontaneous mitotic crossover events in yeast are frequent enough to be an important factor in genome evolution.

  • Received April 24, 2014.
  • Accepted June 20, 2014.