Antimutator Effect Research Articles

Mutator versus antimutator activity of a T4 DNA polymerase mutant distinguishes two different frameshifting mechanisms

Classical "antimutator" DNA polymerases of bacteriophage T4 were examined for their effects upon frameshift mutation rates at a number of positions within rII cistrons. Their antimutagenic activities reduced frameshift frequencies at a number of sites, but at other sites the opposite occurred: the mutant polymerases exhibited clear mutator activities. This dichotomy reveals the operation of two distinct mechanisms of frameshift mutagenesis that are correlated with the DNA sequences at the frameshift sites. Frameshift mutants subject to the antimutator effects of the mutant polymerase lie in A: T-run DNA sequences, where mutations presumably arise by means of the interstrand DNA misalignments postulated by classical theory. The frameshift mutants produced by the mutator activity of these same polymerases lie in quasipalindromic DNA sequences, where mutations are postulated to arise by aberrant metabolism of DNA secondary structures such as hairpins.

Influence of diverse gene 43 DNA polymerases on the incorporation and replication in vivo of 2-aminopurine at A · T base-pairs in bacteriophage T4

The relative rates at which A · T → AP · T † † Abbreviations used: AP, 2-aminopurine; A · T, adenine · thymine base-pair; G · C, guanine · cytosine (5-hydroxymethylcytosine in T4) base-pair; dATP and dAPTP, the deoxynucleoside triphosphates of adenine and 2-aminopurine, respectively; ts, temperature-sensitive. base-pairs are formed in DNA during 2-aminopurine mutagenesis, and the relative rates at which AP · T → AP · C base-pairs are formed during the in vivo replication of templates containing 2-aminopurine, were measured for a mutator, a wild type, and two antimutator bacteriophage T4 DNA polymerases. These measurements did not require quantitation of the 2-aminopurine in the T4 DNA. The mutator polymerase had no discernible effect upon the rate of A · T → AP · T, but increased the rate of AP · T → AP · C twofold over that of the wild-type enzyme. The two antimutator polymerases decreased the rate of formation of both AP · T and AP · C base-pairs compared to the wild-type enzyme. The relative antimutator effect is asymmetrically divided between the two mutation steps producing a very large decrease in the AP · T → AP · C step but a much smaller decrease in the A · T → AP · T step. This and additional observations suggest that the fidelity-enhancing components of T4 DNA polymerases distinguish between 2-aminopurine in template DNA and 2-aminopurine at the primer terminus.

Mutator and antimutator phenotypes of suppressed amber mutants in genes 32, 41, 44, 45, and 62 in bacteriophage T4.

Bacteriophage T4 genes 32, 41, 44, 45, 56, and 62 are essential to DNA replication. Amber mutants (suppressed by su+1, su+2, or su+3 bacteria) in these genes were examined for any mutator or antimutator effects on the reversion of a transition mutation. In every case except for mutations in gene 56, elevated or lowered error frequencies were observed. These results indicate the importance of all of the replicative proteins in the determination of error frequency.

A conditional antimutator inE. coli

A pleiotropic mutation in the purB gene of E. coli is described which lowers the spontaneous mutation frequency of other genes. The antimutator effect is very large for some genetic loci, but is absent at other sites. Both forward and reverse mutations are affected. This mutation in purB is temperature sensitive for both adenine auxotrophy and the antimutator action. Adenine, or adenosine, or low temperature growth abolish the antimutator effect. The mutagenicity of base analogs and nitrosoguanidine at several loci was found to be reduced by this purB mutation. The antimutator effect is recessive in strains merodiploid for the purB region. The frequency of reversion of mutation on F' episomes is affected by the chromosomal antimutator, which therefore acts in trans. Xray and UV sensitivity are normal in this mutant, which is the first antimutator characterized in E. coli.

Stimulation of mutation in phage T 4 by lesions in gene 3 2 and by thymidine imbalance

Summary In bacteriophage T 4 , temperature-sensitive ( ts ) mutations in gene 32 as well as in gene 43 have previously been shown to stimulate reversion of frame-shift mutations. We show here that two ts mutations in gene 32 increase reversion of base substitution mutations. ts mutations in several other genes with DNA-related functions were not observed to have significant mutator or antimutator effects. A recombinational map of gene 32 was constructed using II mutations including the two ts alleles with mutator effects. The length of the map suggests that gene 32 specified a protein of roughly 30 000 molecular weight. It was also shown that a phage mutant with defective thymidylate synthetase has a mutator effect on base substitution mutation when grown under conditions of limiting thymine or thymidine. In addition, these experiments suggested that high levels of exogenous thymine or thymidine stimulate mutation if added to phage with a normal thymidylate synthetase.