We show that self-cleavage of LexA occurs frequently throughout the population during unperturbed growth, rather than being restricted to a subpopulation of cells. These analyses elucidate the mechanisms by which DNA binding and degradation of LexA regulate the SOS response in vivo.
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Here, we developed a single-molecule imaging approach based on a HaloTag fusion to directly monitor LexA in live Escherichia coli cells, demonstrating the existence of three main states of LexA: DNA-bound stationary molecules, free LexA and degraded LexA species. Whether this reflects a population survival strategy or a regulatory inaccuracy is unclear, as are the mechanisms underlying SOS heterogeneity. Considering the fitness burden of these functions, it is surprising that the expression of LexA-regulated genes is highly variable across cells 10, 21, 22, 23 and that cell subpopulations induce the SOS response spontaneously even in the absence of stress exposure 9, 11, 12, 16, 24, 25. SOS induction is also implicated in biofilm formation and antibiotic persistence 11, 18, 19, 20. Self-cleavage of the SOS repressor LexA induces a wide range of cell functions that are critical for survival and adaptation when bacteria experience stress conditions 1 including DNA repair 2, mutagenesis 3, 4, horizontal gene transfer 5, 6, 7, filamentous growth and the induction of bacterial toxins 8, 9, 10, 11, 12, toxin–antitoxin systems 13, virulence factors 6, 14 and prophages 15, 16, 17. The bacterial SOS response represents a paradigm of gene networks controlled by a master transcriptional regulator.