The effect of the SOS response on the mean fitness of unicellular populations: A quasispecies approach

TL;DR

This study models how the SOS response influences the average fitness of unicellular populations, showing it provides a survival advantage when DNA damage is extensive, using an analytically solvable quasispecies framework.

Contribution

It introduces a novel quasispecies model incorporating the SOS response, analyzing its impact on population fitness under DNA damage conditions.

Findings

SOS response confers fitness advantage during extensive DNA damage

Model is analytically solvable for infinite sequence length

Results confirmed by stochastic simulations

Abstract

This paper develops a quasispecies model that incorporates the SOS response. We consider a unicellular, asexually replicating population of organisms, whose genomes consist of a single, double-stranded DNA molecule, i.e. one chromosome. We assume that repair of post-replication mismatched base-pairs occurs with probability $ \lambda $, and that the SOS response is triggered when the total number of mismatched base-pairs exceeds $ l_S $. We further assume that the per-mismatch SOS elimination rate is characterized by a first-order rate constant $ \kappa_{SOS} $. For a single fitness peak landscape where the master genome can sustain up to $ l $ mismatches and remain viable, this model is analytically solvable in the limit of infinite sequence length. The results, which are confirmed by stochastic simulations, indicate that the SOS response does indeed confer a fitness advantage to a population, provided that it is only activated when DNA damage is so extensive that a cell will die if it does not attempt to repair its DNA.