The Escherichia coli DNA glycosylase Mug excises 3,N(4)-ethenocytosines (epsilon 
C) and uracils from DNA, but its biological function is obscure. This is because 
epsilon C is not found in E. coli DNA, and uracil-DNA glycosylase (Ung), a 
distinct enzyme, is much more efficient at removing uracils from DNA than Mug. 
We find that Mug is overexpressed as cells enter stationary phase, and it is 
maintained at a fairly high level in resting cells. This is true of cells grown 
in rich or minimal media, and the principal regulation of mug is at the level of 
mRNA. Although the expression of mug is strongly dependent on the 
stationary-phase sigma factor, sigma(S), when cells are grown in minimal media, 
it shows only a modest dependence on sigma(S) when cells are grown in rich 
media. When mug cells are maintained in stationary phase for several days, they 
acquire many more mutations than their mug(+) counterparts. This is true in ung 
as well as ung(+) cells, and a majority of new mutations may not be C to T. Our 
results show that the biological role of Mug parallels its expression in cells. 
It is expressed poorly in exponentially growing cells and has no apparent role 
in mutation avoidance in these cells. In contrast, Mug is fairly abundant in 
stationary-phase cells and has an important anti-mutator role at this stage of 
cell growth. Thus, Mug joins a very small coterie of DNA repair enzymes whose 
principal function is to avoid mutations in stationary-phase cells.