The human thymine-DNA glycosylase has a sequence homolog in Escherichia coli 
that is described to excise uracils from U.G mismatches (Gallinari, P., and 
Jiricny, J. (1996) Nature 383, 735-738) and is named mismatched uracil 
glycosylase (Mug). It has also been described to remove 3,N(4)-ethenocytosine 
(epsilonC) from epsilonC.G mismatches (Saparbaev, M., and Laval, J. (1998) Proc. 
Natl. Acad. Sci. U. S. A. 95, 8508-8513). We used a mug mutant to clarify the 
role of this protein in DNA repair and mutation avoidance. We find that 
inactivation of mug has no effect on C to T or 5-methylcytosine to T mutations 
in E. coli and that this contrasts with the effect of ung defect on C to T 
mutations and of vsr defect on 5-methylcytosine to T mutations. Even under 
conditions where it is overproduced in cells, Mug has little effect on the 
frequency of C to T mutations. Because uracil-DNA glycosylase (Ung) and Vsr are 
known to repair U.G and T.G mismatches, respectively, we conclude that Mug does 
not repair U.G or T.G mismatches in vivo. A defect in mug also has little effect 
on forward mutations, suggesting that Mug does not play a role in avoiding 
mutations due to endogenous damage to DNA in growing E. coli. Cell-free extracts 
from mug(+) ung cells show very little ability to remove uracil from DNA, but 
can excise epsilonC. The latter activity is missing in extracts from mug cells, 
suggesting that Mug may be the only enzyme in E. coli that can remove this 
mutagenic adduct. Thus, the principal role of Mug in E. coli may be to help 
repair damage to DNA caused by exogenous chemical agents such as 
chloroacetaldehyde.