UV-Induced DNA Repair Mechanisms and Their Effects on Mutagenesis and Culturability inEscherichia coli
Authors: Ghosh Sreyashi , Orman Mehmet A. 
Published: November 2024

Mutagenic processes drive evolutionary progress, with ultraviolet (UV) radiation significantly affecting evolution. Despite extensive research on SOS response-mediated mutagenesis, UV-induced repair mechanisms remain complex, and their effects on cell survival and mutagenesis are not fully understood. We previously observed a near-perfect correlation between RecA-mediated SOS response and mutation levels inEscherichia colifollowing UV treatment. However, prolonged UV exposure caused transient non-culturability and impaired SOS-mediated mutagenesis. Using fluorescent reporters, flow cytometry, promoter-reporter assays, single-gene deletions, knockouts, and clonogenic assays, we found that excessive UV exposure disrupts cellular translation, reducing SOS gene expression, albeit with minimal impact on membrane permeability or reactive oxygen species levels. While our findings underline the abundance of repair mechanisms inE. colicells, enabling them to compensate when specific genes are disrupted, they also highlighted the differential impacts of gene deletions on mutagenesis versus culturability, leading to three major outcomes: (i) Disruption of proteins involved in DNA polymerase for trans-lesion synthesis (UmuC and UmuD) or Holliday junction resolution (RuvC) results in significantly decreased mutagenesis levels while maintaining a transient non-culturability pattern after UV exposure. (ii) Disruption of proteins involved in homologous recombination (RecA and RecB) and nucleotide excision repair (UvrA) leads to both significantly reduced mutagenesis and a more severe transient non-culturability pattern after UV exposure, making these cells more sensitive to UV. (iii) Disruption of DNA Helicase II (UvrD), which functions in mismatch repair, does not affect mutagenesis levels from UV radiation but results in a very pronounced transient non-culturability pattern following UV exposure. Overall, our results further advance our understanding of bacterial adaptation mechanisms and the role of DNA repair pathways in shaping mutagenesis.

Author Summary

Ultraviolet (UV) radiation has been a significant force in driving genetic variation and adaptation throughout billions of years of evolution. By directly damaging DNA and triggering repair mechanisms, UV radiation is a powerful tool for studying mutagenesis. This study aims to provide new insights into the complex processes behind bacterial mutagenesis, a critical topic in microbiology and public health. Although the connection between the SOS response and DNA repair in UV-treated cells is well known, two key phenomena — UV-induced bacterial cell dormancy and mutagenesis — remain poorly understood. Our findings reveal a highly unusual, SOS response-dependent transient non-culturability inEscherichia colicells following prolonged UV exposure. However, the downstream mechanisms of this phenomenon and its links to mutagenesis remain unclear. This study seeks to thoroughly investigate these phenomena, offering new insights into their underlying molecular processes.