Mutation spectrum of spontaneous frameshift revertants in yeast using double-strand gap repair

A mutation spectrum was constructed from a series of randomly isolated spontaneous His+ revertants of the frameshift mutant his4-38 in Saccharomyces cerevisiae. For each true revertant, a 438 bp region encompassing his4-38 on chromosome III was recovered into a shuttle vector by double-strand gap repair. Of the 45 independent His+ revertants sequenced, 44 were -1 base deletions and one revertant was a +2 base insertion. The -1 deletions exhibited a bimodal distribution. Of the bases encompassing the his4-38 region from +153-181, approximately 45% were not involved in a reversion event, although a -1 frameshift within this region will result in a viable His+ revertant. Approximately 49% of -1 events occurred within runs of 3 repeated bases. At these sites the strand-slippage model for frameshift mutation is supported. However, the -1 events occurring at sites of 2 repeated bases and the low frequency (2%) of +2 base insertions suggest that the transiently misaligned template model is a significant mechanism in reversion of his4-38. When the distribution of -1 events at repeated bases was discounted, a hotspot involving a -T at position +163 was resolved.