Role of RecA protein in untargeted UV mutagenesis of bacteriophage lambda: evidence for the requirement for the dinB gene.

Untargeted UV mutagenesis of bacteriophage lambda--i.e., the increased recovery of lambda mutants when unirradiated lambda infects UV-irradiated Escherichia coli--is thought to be mediated by a transient decrease in DNA replication fidelity, generating mutations in the newly synthesized strands. Using the bacteriophage lambda cI857----lambda c mutation system, we provide evidence that the RecA protein, shown previously to be required for this mutagenic pathway, is no longer needed when the LexA protein is inactivated by mutation. We suggest that the error-prone DNA replication responsible for UV-induced untargeted mutagenesis is turned on by the presence of replication-blocking lesions in the host cell DNA and that the RecA protein is required only to derepress the relevant din gene(s). This is in contrast to mutagenesis of irradiated bacteria or irradiated phage lambda, in which activated RecA protein has a second role in mutagenesis in addition to the cleavage of the LexA protein. Among the tested din genes, the dinB gene product (in addition to the uvrA and uvrB gene products) was found to be required for untargeted mutagenesis of bacteriophage lambda. To our knowledge, a phenotype associated with the dinB gene has not been reported previously.