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J. Bacteriol., Dec 1997, 7435-7445, Vol 179, No. 23
IJ Fijalkowska, RL Dunn and RM Schaaper
To better understand the mechanisms of SOS mutagenesis in the bacterium
Escherichia coli, we have undertaken a genetic analysis of the SOS mutator
activity. The SOS mutator activity results from constitutive expression of
the SOS system in strains carrying a constitutively activated RecA protein
(RecA730). We show that the SOS mutator activity is not enhanced in strains
containing deficiencies in the uvrABC nucleotide excision-repair system or
the xth and nfo base excision- repair systems. Further, recA730-induced
errors are shown to be corrected by the MutHLS-dependent mismatch-repair
system as efficiently as the corresponding errors in the rec+ background.
These results suggest that the SOS mutator activity does not reflect
mutagenesis at so-called cryptic lesions but instead represents an
amplification of normally occurring DNA polymerase errors. Analysis of the
base-pair- substitution mutations induced by recA730 in a mismatch repair-
deficient background shows that both transition and transversion errors are
amplified, although the effect is much larger for transversions than for
transitions. Analysis of the mutator effect in various dnaE strains,
including dnaE antimutators, as well as in proofreading- deficient dnaQ
(mutD) strains suggests that in recA730 strains, two types of replication
errors occur in parallel: (i) normal replication errors that are subject to
both exonucleolytic proofreading and dnaE antimutator effects and (ii)
recA730-specific errors that are not susceptible to either proofreading or
dnaE antimutator effects. The combined data are consistent with a model
suggesting that in recA730 cells error-prone replication complexes are
assembled at sites where DNA polymerization is temporarily stalled, most
likely when a normal polymerase insertion error has created a poorly
extendable terminal mismatch. The modified complex forces extension of the
mismatch largely at the exclusion of proofreading and polymerase
dissociation pathways. SOS mutagenesis targeted at replication-blocking DNA
lesions likely proceeds in the same manner.
Copyright © 1997, American Society for Microbiology
Genetic requirements and mutational specificity of the Escherichia coli SOS mutator activity
Laboratory of Molecular Genetics, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709, USA.
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