New complexities of SOS-induced “untargeted” mutagenesis in Escherichia coli as revealed by mutation accumulation and whole-genome sequencing

• Published in: DNA repair Vol. 90; p. 102852
• Main Authors: Niccum, Brittany A., Coplen, Christopher P., Lee, Heewook, Mohammed Ismail, Wazim, Tang, Haixu, Foster, Patricia L.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.06.2020
• Subjects: DNA polymerase fidelity; DNA, Bacterial - metabolism; DNA-Directed DNA Polymerase - metabolism; Error-prone DNA polymerases; Escherichia coli - genetics; Mutagenesis; Mutation; Mutation accumulation; Mutation hotspots; Mutation Rate; SOS mutagenesis; SOS response; SOS Response, Genetics; Whole Genome Sequencing; SOS mutagenesis; DNA polymerase fidelity; Mutation hotspots; SOS response; Mutation accumulation; Error-prone DNA polymerases
• ISSN: 1568-7864; 1568-7856; 1568-7856
• DOI: 10.1016/j.dnarep.2020.102852

•The SOS response to DNA damage induces “untargeted” mutations.•SOS-mutations are revealed by mutation accumulation and whole genome sequencing.•SOS-mutations are both sequence and DNA-strand biased.•G:C to C:G transversions are particularly highly induced by SOS.•G:C to C:G transversions are extremely sequence and DNA-strand biased. When its DNA is damaged, Escherichia coli induces the SOS response, which consists of about 40 genes that encode activities to repair or tolerate the damage. Certain alleles of the major SOS-control genes, recA and lexA, cause constitutive expression of the response, resulting in an increase in spontaneous mutations. These mutations, historically called “untargeted”, have been the subject of many previous studies. Here we re-examine SOS-induced mutagenesis using mutation accumulation followed by whole-genome sequencing (MA/WGS), which allows a detailed picture of the types of mutations induced as well as their sequence-specificity. Our results confirm previous findings that SOS expression specifically induces transversion base-pair substitutions, with rates averaging about 60-fold above wild-type levels. Surprisingly, the rates of G:C to C:G transversions, normally an extremely rare mutation, were induced an average of 160-fold above wild-type levels. The SOS-induced transversion showed strong sequence specificity, the most extreme of which was the G:C to C:G transversions, 60% of which occurred at the middle base of 5′GGC3′+5′GCC3′ sites, although these sites represent only 8% of the G:C base pairs in the genome. SOS-induced transversions were also DNA strand-biased, occurring, on average, 2- to 4- times more often when the purine was on the leading-strand template and the pyrimidine on the lagging-strand template than in the opposite orientation. However, the strand bias was also sequence specific, and even of reverse orientation at some sites. By eliminating constraints on the mutations that can be recovered, the MA/WGS protocol revealed new complexities of SOS “untargeted” mutations.