The effect of hyper-induced SOS response on evolution in Escherichia coli

Background The SOS response, a conserved bacterial DNA repair system, plays a critical role in adaptation under stress, by inducing spontaneous mutations. This study compared the evolutionary dynamics of Escherichia coli and a derivative of this strain with a constitutively activated SOS response (SOS*), focusing on growth phenotype, competitive fitness, genome evolution, and metabolism over 90 days of evolution.

Results Laboratory evolution revealed that both wild-type and SOS* populations increased in growth rates, reduced average cell lengths, improved competitive fitness, and expanded their catabolic substrate utilization over time. However, SOS* strains, which revealed initial reduced growth rate, fitness and carbon-utilization, adapted at a faster rate than the wild-type, and obtained levels comparable to the wild-type. Whole-genome sequencing identified higher mutation rates and distinct mutation patterns in the SOS* populations compared to the wild-type populations, particularly an increase in non-synonymous substitutions, suggesting positive selection for adaptive traits. Mutational analyses revealed convergence in regulatory and metabolic pathways in both wild-type and SOS* groups. However, SOS* accumulated a higher number of mutations in regulatory and metabolic genes, consistent with increased phenotypic flexibility during adaptation. Despite this, their fitness and carbon source utilization abilities did not surpass those of the evolved wild-types.

Conclusion Our results indicate that while SOS-mediated mutagenesis enhances genetic diversity and accelerates adaptation, its long-term impact on adaptive evolution may be limited under non-selective environments due to the sustained SOS activation.