Chronology in lesion tolerance gives priority to genetic variability

The encounter of a replication fork with a blocking DNA lesion is a common event that cells need to address properly to preserve genome integrity. Cells possess two main strategies to tolerate unrepaired lesions: potentially mutagenic translesion synthesis (TLS) and nonmutagenic damage avoidance (DA). Little is known about the partitioning between these two strategies. Because genes involved in DA mechanisms (i.e., recA) are expressed early and genes involved in TLS (i.e., Pol V) are expressed late during the bacterial SOS response, it has long been thought that TLS was the last recourse to bypass DNA lesions when repair and nonmutagenic DA mechanisms have failed. By using a recently described methodology, we followed the fate of a single replication-blocking lesion introduced in the Escherichia coli genome during acute genotoxic stress. We show that lesion tolerance events (i) only occur when the SOS response is fully induced and (ii) are executed in chronological order, with TLS coming first, followed by DA. Therefore, in response to genotoxic stress, bacterial cells give priority to TLS, a minor pathway able to generate genetic diversity before implementing the major nonmutagenic pathway that ensures survival.