Nucleotide pool sanitizing enzymes Dut (dUTPase), RdgB (dITPase), and MutT 
(8-oxo-dGTPase) of Escherichia coli hydrolyze noncanonical DNA precursors to 
prevent incorporation of base analogs into DNA. Previous studies reported 
dramatic AT-->CG mutagenesis in mutT mutants, suggesting a considerable density 
of 8-oxo-G in DNA that should cause frequent excision and chromosomal 
fragmentation, irreparable in the absence of RecBCD-catalyzed repair and similar 
to the lethality of dut recBC and rdgB recBC double mutants. In contrast, we 
found mutT recBC double mutants viable with no signs of chromosomal 
fragmentation. Overproduction of the MutM and MutY DNA glycosylases, both acting 
on DNA containing 8-oxo-G, still yields no lethality in mutT recBC double 
mutants. Plasmid DNA, extracted from mutT mutM double mutant cells and treated 
with MutM in vitro, shows no increased relaxation, indicating no additional 
8-oxo-G modifications. Our DeltamutT allele elevates the AT-->CG transversion 
rate 27,000-fold, consistent with published reports. However, the rate of 
AT-->CG transversions in our mutT(+) progenitor strain is some two orders of 
magnitude lower than in previous studies, which lowers the absolute rate of 
mutagenesis in DeltamutT derivatives, translating into less than four 8-oxo-G 
modifications per genome equivalent, which is too low to cause the expected 
effects. Introduction of various additional mutations in the DeltamutT strain or 
treatment with oxidative agents failed to increase the mutagenesis even twofold. 
We conclude that, in contrast to the previous studies, there is not enough 
8-oxo-G in the DNA of mutT mutants to cause elevated excision repair that would 
trigger chromosomal fragmentation.