We have measured reversion rates of
specific base pair mutations in the presence and absence of mismatch
repair in S. cerevisiae . Our assay uses six isogenic strains each
with a different point mutation in the CYC1 gene that renders the
protein non-functional. Only true revertants give a Cyc+ phenotype. We
have deleted MSH2 , MSH3 , and MSH6 separately in
each of these strains. The reversion rates of three strains were not
increased in the absence of mismatch repair. The reversion rates of the
other three strains were increased nearly 1000-fold or more in the
absence of either MSH2 or MSH6 . We hypothesize that the
high reversion rates are due to misincorporations across from damaged
bases. To determine if reversion rates were affected by oxidative
damage, we grew the strains with high reversion rates in an anaerobic
environment, which decreased the reversion rates of the three strains as
much as 100-fold. The mutations that give rise to the reversions in
these strains are commonly found in cells under oxidative attack. These
results suggest that a major function of the mismatch repair system in
yeast is to prevent mutations due to misincorporation across from at
least some damaged bases. We suggest that this function and the
prevention of mutations due to slipped mispairing are the two major
repair functions for mismatch repair and that the repair of single base
mismatches in undamaged DNA is a relatively minor role.
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