Spontaneous mutagenesis in O6-alkylguanine-DNA alkyltransferase-proficient and 
-deficient (ada ogt mutants) Escherichia coli was studied in two ways: in 
bacteria growing in nonselective liquid medium and in bacteria resting on 
selective agar plates. ATase mutants showed similar spontaneous mutation rates 
as ATase proficient bacteria during growth phase; an excess of mutants arising 
in nondividing cells. The resting-associated mutagenesis in ada + ogt + uvr- 
bacteria was biphasic; the high sensitive range being triggered beyond the first 
6 days after plating. Contrarily, spontaneous Lacc mutants from ada- ogt- uvr- 
cells steadily increased over the 8 day period of plate incubation. These 
results suggested that, in the absence of nucleotide excision repair, the repair 
by both the Ada and the Ogt ATases is not saturated until the cells have been 
resting for 6 days. The spontaneous LacI-d mutation spectrum of ada + ogt + uvr- 
bacteria growing in non-selective liquid medium served as a baseline to 
determine the mutation events increased in the ATase-deficient derivative upon 
prolonged incubation on selective plates. The percentage of G:C-->A:T 
transitions, presumably driven by unrepaired O6-alkylguanine lesions, was 
increased at the expense of other mutation types. G:C-->A:T transitions 
accumulated with a pronounced 5'PuG bias, suggesting that the endogenous 
metabolite(s) responsible for this mutation class is an SN1 type alkylating 
compound(s). Accordingly, the site distribution of G:C-->A:T transitions in 
nondividing ATase defective bacteria showed similarities with the spectra 
induced by alkylnitrosoureas, particularly with those generating bulky alkylated 
DNA adducts.