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Deoxyribosyl-dihydropyrimido [4,5-c] [1,2] oxazin-7-one (dP) is a potent mutagenic deoxycytidine-derived base analogue capable of pairing with both A and G, thereby causing G * C [right arrow] A * T and A * T [right arrow] G * C transition mutations. We have found that the Escherichia coli DNA mismatch-repair system can protect cells against this mutagenic action. At a low dose, dP is much more mutagenic in mismatch-repair-defective mutH, mutL, and mutS strains than in a wild-type strain. At higher doses, the difference between the wild-type and the mutator strains becomes small, indicative of saturation of mismatch repair. Introduction of a plasmid containing the E. coli mut[L.sup.+] gene significantly reduces dP-induced mutagenesis. Together, the results indicate that the mismatch-repair system can remove dP-induced replication errors, but that its capacity to remove dP-containing mismatches can readily be saturated. When cells are cultured at high dP concentration, mutant frequencies reach exceptionally high levels and viable cell counts are reduced. The observations are consistent with a hypothesis in which dP-induced cell killing and growth impairment result from excess mutations (error catastrophe), as previously observed spontaneously in proofreading-deficient mutD (dnaQ) strains.

Antimutators are mutant strains that have reduced mutation rates compared to the corresponding wild-type strain. Their existence, along with mutator mutants that have higher mutation rates compared to the wild-type strain, are powerful evidence that mutation rates are genetically controlled. Compared to mutator mutants, antimutators have a very distinguishing property. Because they prevent normally occurring mutations, they, uniquely, are capable of providing insight into the mechanisms of spontaneous mutations. In this review, antimutator mutants are discussed in bacteriophage T4 and the bacterium Escherichia coli, with regard to their properties, possible mechanisms, and implications for the sources of spontaneous mutations in these two organisms.

DNA sequencing and colony hybridization studies of the function of the dnaE911 DNA polymerase antimutator allele in spontaneous mutagenesis of Escherichia coli reveal that dnaE911 reduces galK, lacI and Rif mutations by 1.5 to 2-fold in the mismatch-repair proficient background. DNA sequences of the lacI gene mutants show that transitions, deletions, frameshifts and duplications remain stable, while a 3-fold reduction of transversions occurs in the dnaE911 allele presence. The A.T to T.A transversion undergoes a 6-fold reduction while the A.T to G.C base-pair substitution hotspot in the lac operator undergoes 5-fold decrease, suggesting that dnaE911 allelic activity is genetic background and mutation type specific.

The mutations in approximately 2400 4-aminobiphenyl (4-AB) + S9-induced revertants of the -1 frameshift allele hisD3052 and the base-substitution allele hisG46 of Salmonella typhimurium were analyzed using colony probe hybridization and polymerase chain reaction. Around 85% of the revertants had the hotspot deletion, while around 15% had complex frameshifts. This was evident in strain TA98 (DeltauvrB, pKM101) which bore the SOS DNA repair system of the pKM101 plasmid. These mutations are cryptic complex frameshifts.

Specific mutants of Escherichia coli were isolated and characterized by specific E. coli antimutators. The mutants were derived by the mutagenesis of the dnaE and dnaQ areas of the chromosome. Each of the mutants were found to be present in the dnaE gene which encodes the polymerase subunit of the DNA polymerase III holoenzyme. The mutants were isolated in a mutL strain, in which most of them were derived as a result of DNA replication errors. It is suggested that the isolated mutant of E. coli display increased accuracy during in vivo DNA replication.

Seven antimutator mutants of Escherichia coli were seen to possess lower mutation rates than the wild-type strains. The dnaE gene of E. coli encodes the DNA polymerase of the primary replicative enzyme, which is the DNA polymerase III holoenzyme. This gene decreases errors in DNA replication. The nucleotide sequence differs with each antimutator dnaE alleles. The function of the polymerase III, which contains the affected residues from amino acid alignment with the other DNA polymerase, was also revealed.

A series of Escherichia coli mutants were isolated and their properties studied after proving that these mutants carried mutation in the dnaE gene. These mutants can replicate their DNA with high fidelity. The mutation in the dnaE gene, which these mutants carry, encodes the subunit of DNA polymerase III holoenzyme, which replicates the bacterial chromosomes. The mutants also act as suppressors of high mutability mutL strain, in which mutations are traced to errors in the DNA replication.