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, 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. © 1992 Wiley‐Liss, Inc.