The impact of lagging strand replication mutations on the stability of CAG repeat tracts in yeast

We have examined the stability of long tracts of CAG repeats in yeast mutants defective in enzymes suspected to be involved in lagging strand replication. Alleles of DNA ligase (cdc9-1 and cdc9-2) destabilize CAG tracts in the stable tract orientation, i.e., when CAG serves as the lagging strand template. In this orientation nearly two-thirds of the events recorded in the cdc9-1 mutant were tract expansions. While neither DNA ligase allele significantly increases the frequency of tract-length changes in the unstable orientation, the cdc9-1 mutant produced a significant number of expansions in tracts of this orientation. A mutation in primase (pri2-1) destabilizes tracts in both the stable and the unstable orientations. Mutations in a DNA helicase/deoxyribonuclease (dna2-1) or in two RNase H activities (rnh1Δ and rnh35Δ) do not have a significant effect on CAG repeat tract stability. We interpret our results in terms of the steps of replication that are likely to lead to expansion and to contraction of CAG repeat tracts.