The transcription elongation factor NusA is required for stress-induced mutagenesis in Escherichia coli

Genetic adaptation: a new piece for a very old puzzle

Does stress create mutations or only serve as an agent of natural selection? New experiments reveal effects of transcription and temperature on the response to growth limitation and could help resolve a 150-year-old debate.

The sigma(E) stress response is required for stress-induced mutation and amplification in Escherichia coli

Pathways of mutagenesis are induced in microbes under adverse conditions controlled by stress responses. Control of mutagenesis by stress responses may accelerate evolution specifically when cells are maladapted to their environments, i.e. are stressed. Stress-induced mutagenesis in the Escherichia coli Lac assay occurs either by ‘point’ mutation or gene amplification. Point mutagenesis is associated with DNA double-strand-break (DSB) repair and requires DinB error-prone DNA polymerase and the SOS DNA-damage- and RpoS general-stress responses. We report that the RpoE envelope-protein-stress response is also required. In a screen for mutagenesis-defective mutants, we isolated a transposon insertion in the rpoE P2 promoter. The insertion prevents rpoE induction during stress, but leaves constitutive expression intact, and allows cell viability. rpoE insertion and suppressed null mutants display reduced point mutagenesis and maintenance of amplified DNA. Furthermore, σ^E acts independently of stress responses previously implicated: SOS/DinB and RpoS, and of σ³², which was postulated to affect mutagenesis. I-SceI-induced DSBs alleviated much of the rpoE phenotype, implying that σ^E promoted DSB formation. Thus, a third stress response and stress input regulate DSB-repair-associated stress-induced mutagenesis. This provides the first report of mutagenesis promoted by σ^E, and implies that extracytoplasmic stressors may affect genome integrity and, potentially, the ability to evolve.

RpoS, the stress response sigma factor, plays a dual role in the regulation of Escherichia coli's error-prone DNA polymerase IV

RpoS, Escherichia coli's general stress response sigma factor, regulates error-prone DNA polymerase IV (Pol IV) (encoded by the dinB gene). Pol IV is induced in stationary-phase cells, and thereafter, levels of the protein remain elevated for several days of continuous incubation. This induction and persistence in stationary-phase cells are dependent on RpoS. Data presented here show that this regulation is direct via the RpoS-directed transcription of the dinB gene. However, a loss of RpoS also results in a decrease in Pol IV-dependent mutation when Pol IV is overexpressed from an RpoS-independent promoter in exponentially growing cells. The loss of RpoS also increases cell sensitivity to 4-nitroquinoline-1-oxide, indicating that RpoS affects the ability of Pol IV to bypass DNA lesions. Thus, in addition to directly driving the transcription of the dinB gene in stationary-phase cells, RpoS regulates the activity of Pol IV in exponentially growing cells via a second, indirect pathway.

Transcription-associated mutation in Bacillus subtilis cells under stress

Adaptive (stationary phase) mutagenesis is a phenomenon by which nondividing cells acquire beneficial mutations as a response to stress. Although the generation of adaptive mutations is essentially stochastic, genetic factors are involved in this phenomenon. We examined how defects in a transcriptional factor, previously reported to alter the acquisition of adaptive mutations, affected mutation levels in a gene under selection. The acquisition of mutations was directly correlated to the level of transcription of a defective leuC allele placed under selection. To further examine the correlation between transcription and adaptive mutation, we placed a point-mutated allele, leuC427, under the control of an inducible promoter and assayed the level of reversion to leucine prototrophy under conditions of leucine starvation. Our results demonstrate that the level of Leu(+) reversions increased significantly in parallel with the induced increase in transcription levels. This mutagenic response was not observed under conditions of exponential growth. Since transcription is a ubiquitous biological process, transcription-associated mutagenesis may influence evolutionary processes in all organisms.