Dynamics of the inducing signal for the SOS regulatory system in Escherichia coli after ultraviolet irradiation

SOS response in Escherichia coli is induced by various DNA-damaging treatments, for example by ultraviolet irradiation, to help a cell to recover from the damage. During induction of the SOS regulatory system, generation of the inducing signal for the system is the early step. In the present study a model for quantitative description of the signal dynamics is developed. We derive the inducing signal, in terms of concentration of single-stranded DNA, as a function of time since the moment of ultraviolet irradiation. Simulation of the signal level after irradiation with two doses of 5 and 20 J m−2 is presented. This provides quantitative description of the event that controls various cellular physiological reactions induced in the course of the SOS response. The dynamics of the signal level are then used as an input for a dynamical equation description of the SOS regulatory system that we proposed earlier. This allows for a quantitative analysis of the subsequent step in the SOS induction: cleavage of LexA protein, a negative regulator of the SOS system. The model is verified against available experimental data for LexA protein level in ultraviolet radiation-induced Escherichia coli cells.