Figure 4.

The SOS response is an inducible DNA repair pathway controlled by two key regulators, the LexA repressor of gene expression and the RecA inducer. Under normal conditions, LexA is bound to the promoter of SOS-associated genes preventing binding of RNA polymerase (RNA Pol) and expression of the regulated genes (Maslowska et al., 2019). Following DNA damage, such as following certain antimicrobial exposures, the DNA polymerase III (DNA Pol III) enzyme senses the damage and the increase in single-stranded DNA (ssDNA) causes RecA to become activated and bind to the ssDNA. The ssDNA/RecA complex leads to proteolysis of the LexA repressor allowing RNA polymerase to bind facilitating expression of genes under LexA control. LexA has been shown to impact the expression of integrases and transposases, as such the activation of the SOS response has been shown to promote horizontal dissemination of resistance genes. Additionally, conjugation can also trigger the SOS response as ssDNA is generated and transferred during the conjugation process which can upregulate integrase and transposase expression and triggering recombination of plasmids in the recipient cells.