Mutagenesis of lambda phage by tif-expression or host-irradiation functions is largely independent of damage in the phage DNA

Hypermutation in bacteria and other cellular systems

A temporary state of hypermutation can in principle arise through an increase in the rate of polymerase errors (which may or may not be triggered by template damage) and/or through abrogation of fidelity mechanisms such as proofreading and mismatch correction. In bacteria there are numerous examples of transient mutator states, often occurring as a consequence of stress. They may be targeted to certain regions of the DNA, for example by transcription or by recombination. The initial errors are made by various DNA polymerases which vary in their error-proneness: several are inducible and are under the control of the SOS system. There are several structurally related polymerases in mammals that have recently come to light and that have unusual properties, such as the ability to carry out 'accurate' translesion synthesis opposite sites of template damage or the possession of exceedingly high misincorporation rates. In bacteria the initial errors may be genuinely spontaneous polymerase errors or they may be triggered by damage to the template strand, for example as a result of attack by active oxidative species such as singlet oxygen. In mammalian cells, hypermutable states persisting for many generations have been shown to be induced by various agents, not all of them DNA damaging agents. A hypermutable state induced by ionizing radiation in male germ cells in the mouse results in a high rate of sequence errors in certain unstable minisatellite loci; the mechanism is unclear but believed to be associated with recombination events.

Relationship between cellular RecA protein concentration and untargeted mutagenesis in Escherichia coli

We measured the production of untargeted mutations in the cI and cII genes of untreated lambda phage undergoing a lytic cycle in UV-irradiated bacterial hosts. As previously shown, treatment with 4 micrograms/ml of rifampicin during post-irradiation incubation inhibited amplification of the RecA protein in these cells. In addition, we observed a decreased mutation rate compared to the untreated, irradiated bacteria. Treatment with 4 micrograms/ml or 8 micrograms/ml rifampicin did not prevent the UV induction of the umuDC operon, as judged by assay of beta-galactosidase activity in a umuC-lacZ fusion strain. In contrast, the UV-induction of beta-galactosidase in the sulA-lacZ fusion strain was decreased by 4 micrograms/ml rifampicin. The inhibition of untargeted mutagenesis by this drug treatment was also observed in a strain constitutive for SOS functions (lexA (Def)) as well as in a RecA-overproducing plasmid strain, suggesting the requirement of other factor(s) in wild-type recA+ cells. An htpR165-carrying strain, that blocks induction of heat-shock proteins, exhibited normal UV-promoted mutagenesis. A correlation was observed between the cellular concentration of RecA protein, increased spontaneously by a temperature shift in a lexA(Ts) strain, and the extent of UV-promoted untargeted mutagenesis. These results suggest a mechanistic role of RecA protein in this process.

UV-induced imbalance of the deoxyribonucleoside triphosphate pool in E. coli

The effect of UV irradiation on the intracellular DNA precursor pool in E. coli was investigated. UV irradiation of E. coli, followed by post-incubation for 1-1.5 h, altered the relative sizes of the deoxyribonucleoside triphosphate (dNTP) pool. The total amount of dNTPs increased: both dATP and dTTP increased several-fold, dCTP about twofold, while dGTP remained almost unchanged. In recA- and umuC- strains, which are defective in UV-induced mutagenesis, the pattern of nucleotide pool alterations was similar to that of wild-type strains.

Further evidence that ultraviolet radiation-enhanced reactivation of simian virus 40 in monkey kidney cells is not accompanied by mutagenesis

Can simian virus 40 (SV40) be used to detect mutagenic DNA repair in cultured mammalian cells? The published evidence from different laboratories are in direct conflict. In order to decide between the conflicting evidence, we conducted experiments in two separate laboratories using experimental protocols similar to those previously used to investigate mutagenic repair with viral probes. Mutagenesis in SV40 virus stocks obtained by infecting ultraviolet (UV)-irradiated or unirradiated CV-1 monkey kidney cells with UV-irradiated or unirradiated temperature-sensitive SV40 mutant tsB201 was investigated. The frequency of reversion of the ts mutant to phenotypically wild-type virus was determined by assaying the virus stocks at permissive (33 degrees) and non-permissive (39 degrees) temperatures. These data show that (a) the reversion frequency for unirradiated virus propagated in irradiated cells was more than that in unirradiated cells; (b) irradiated virus gave more reversion than unirradiated virus in unirradiated and irradiated cells; and (c) irradiated virus had a lower reversion frequency in irradiated cells than in unirradiated cells. Reactivation experiments carried out in parallel; with the mutagenesis showed enhanced reactivation in UV-irradiated SV40 in UV-irradiated CV-1 cells. We conclude that enhanced reactivation of UV-irradiated SV40 was not mutagenic in monkey kidney cells.

Constitutive error-prone repair at sites of excision repair in Escherichia coli: a re-examination

No evidence was found for production during incubation in chloramphenicol of a signal capable of inducing a rec-lex function (lysogenic induction) when protein synthesis was restored. The majority of UV-induced mutations to streptomycin resistance require lesions in the DNA and cannot be attributed to untargeted mutator activity. Therefore the loss of photoreversibility of these mutations occurring in the absence of protein synthesis and DNA replication cannot be attributed to the production of a signal for the induction of subsequent rec-lex dependent mutator activity. The simplest interpretation is that they arise as errors at excision sites by the operation of a constitutive pathway.