DinB Upregulation Is the Sole Role of the SOS Response in Stress-Induced Mutagenesis inEscherichia coli

Abstract

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(o c ) (operator-constitutive) mutants. Both produce SOS-induced levels of DinB constitutively. We find that both dinB(o c ) alleles fully suppress the phenotype of constitutively SOS-''off'' lexA(Ind À ) 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 of dinB by the dinB(o c ) 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. G ENOMIC stability and mutation rates are tightly regulated features of all organisms. Understanding how cells regulate mutation rates has important implications for evolution, cancer progression and chemotherapy resistance, aging, and acquisition of antibiotic resistance and evasion of the immune system by pathogens, all processes driven by mutagenesis and all of which occur during stress.Stress-induced mutagenesis refers to a group of related phenomena in which cells poorly adapted to their environment (i.e., stressed) increase mutation rates as part of a regulated stress response (reviewed by Galhardo et al. 2007). Abundant examples, particularly in microorganisms, show the induction of specific pathways of mutagenesis in response to stresses. The types of genetic alteration induced by stress include base substitutions, small deletions and insertions, gross chromosomal rearrangements and copy-number variations, and movement of mobile elements. These various pathways require the functions of different sets of genes and proteins. Thus, there appear to be multiple molecular mechanisms of stress-inducible mutagenesis that operate in different organisms, cell types, and growthinhibiting stress conditions. However, a common theme in the many mechanisms of stress-inducible mutagenesis described to date is the requirement for the function of one or more cellular stress responses. Starvation stress-induced mutagenesis in Bacillus subtilis re...

Divergent Roles for the Two PolI-Like Organelle DNA Polymerases of Arabidopsis

“…The large increase of MMBIR rearrangements in ptDNA observed in the polIb-1/why1why3 mutant in the absence of genotoxic stress supports this conclusion and indicates that PolIB is not required or essential for DNA repair by MMBIR. This is reminiscent of the situation in yeast (Saccharomyces cerevisiae) where break-induced replication as well as MMBIR are dependent on a nonessential subunit of Pold called POL32 (Lydeard et al, 2007;Payen et al, 2008) and in Escherichia coli where it is dependent on DinB (Galhardo et al, 2009). Our results suggest that, in the absence of PolIB, conservative repair of DSBs is reduced, leading to the accumulation of substrates (DSBs) that are repaired by the error-prone MMBIR in absence of Whirlies.…”

Antibiotic treatment enhances the genome-wide mutation rate of target cells

“…Three error-prone DNA polymerases are expressed in Escherichia coli during the SOS response, substantially elevating the mutation rate (21,22). Under sustained exposure to norfloxacin, mutation rates revealed by reporter constructs can be elevated 2-9 times (18,23).…”

Roles of Nucleoid-Associated Proteins in Stress-Induced Mutagenic Break Repair in StarvingEscherichia coli

“…RpoS licenses the use of DinB and other error-prone DNA polymerases in break repair by an unknown mechanism (Ponder et al 2005;Frisch et al 2010;Shee et al 2011). The SOS DNA-damage response is required for SNV formation (McKenzie et al 2000) and promotes MBR by its induction of the DinB error-prone DNA polymerase (McKenzie et al 2001;Galhardo et al 2009). Neither SOS nor DinB plays a role in GCR formation (McKenzie et al 2001).…”