The effect of DNA replication mutations on CAG tract stability in yeast

CAG repeat tracts are unstable in least, leading to frequent contractions a nd infrequent expansions in repeat tract length. To compare CAG repeats to other simple repeats and palindromic sequences, we examined the effect of D NA replication mutations, including alleles of pol alpha, pol delta, pol ep silon, and PCNA (proliferating cell nuclear antigen), on tract stability. A mong the polymerase mutations, the pol delta mutation (pol3-14 destabilizes tracts with either CAG or CTG as the lagging strand template. One pol alph a mutation, pol1-1, destabilizes the orientation with CAG as the lagging st rand template, but it has little effect on the CTG orientation. In contrast , thr pol1-17 mutation has no effect on either orientation. Similarly, muta tions in the proofreading functions of pol delta and pol epsilon, as well a s a temperature-sensitive pol epsilon mutation, pol2-18, have no effect on tract stability. Three PCNA mutations, pol30-52, pol30-79 and pol30-90, all have drastic effects on tract stability. Of the three, pol30-52 is unique in yielding small tract changes that are indicative of an impairment in mis match repair. These results show that while CAG repeats are destabilized by many of the same mutations that destabilize other simple repeats, they als o have some behaviors that are suggestive of their potential to form hairpi n structures.