Stress-induced mutagenesis is a collection of mechanisms observed in bacterial, yeast, and human cells in
which adverse conditions provoke mutagenesis, often under the control of stress responses. Control of
mutagenesis by stress responses may accelerate evolution specifically when cells are maladapted to their
environments, i.e., are stressed. It is therefore important to understand how stress responses increase
mutagenesis. In the Escherichia coli Lac assay, stress-induced point mutagenesis requires induction of
at least two stress responses: the RpoS-controlled general/starvation stress response and the SOS DNAdamage response, both of which upregulate DinB error-prone DNA polymerase, among other genes
required for Lac mutagenesis. We show that upregulation of DinB is the only aspect of the SOS response
needed for stress-induced mutagenesis. We constructed two dinB(oc
) (operator-constitutive) mutants. Both
produce SOS-induced levels of DinB constitutively. We find that both dinB(oc
) alleles fully suppress the
phenotype of constitutively SOS-‘‘off’’ lexA(Ind
) mutant cells, restoring normal levels of stress-induced
mutagenesis. Thus, dinB is the only SOS gene required at induced levels for stress-induced point
mutagenesis. Furthermore, although spontaneous SOS induction has been observed to occur in only a small
fraction of cells, upregulation ofdinBby thedinB(oc
) alleles in all cells does not promote a further increase in
mutagenesis, implying that SOS induction of DinB, although necessary, is insufficient to differentiate cells
into a hypermutable condition
Stress-induced mutagenesis is a collection of mechanisms observed in bacterial, yeast, and human cells in
which adverse conditions provoke mutagenesis, often under the control of stress responses. Control of
mutagenesis by stress responses may accelerate evolution specifically when cells are maladapted to their
environments, i.e., are stressed. It is therefore important to understand how stress responses increase
mutagenesis. In the Escherichia coli Lac assay, stress-induced point mutagenesis requires induction of
at least two stress responses: the RpoS-controlled general/starvation stress response and the SOS DNAdamage response, both of which upregulate DinB error-prone DNA polymerase, among other genes
required for Lac mutagenesis. We show that upregulation of DinB is the only aspect of the SOS response
needed for stress-induced mutagenesis. We constructed two dinB(oc
) (operator-constitutive) mutants. Both
produce SOS-induced levels of DinB constitutively. We find that both dinB(oc
) alleles fully suppress the
phenotype of constitutively SOS-‘‘off’’ lexA(Ind
) mutant cells, restoring normal levels of stress-induced
mutagenesis. Thus, dinB is the only SOS gene required at induced levels for stress-induced point
mutagenesis. Furthermore, although spontaneous SOS induction has been observed to occur in only a small
fraction of cells, upregulation ofdinBby thedinB(oc
) alleles in all cells does not promote a further increase in
mutagenesis, implying that SOS induction of DinB, although necessary, is insufficient to differentiate cells
into a hypermutable condition
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