DNA lesions proximity modulates damage tolerance pathways in Escherichia coli

The genome of all organisms is constantly threatened by numerous agents that cause DNA damage. When the replication fork encounters an unrepaired DNA lesion, two DNA damage tolerance pathways are possible: error-prone translesion synthesis (TLS) that requires specialized DNA polymerases, and error-free damage avoidance that relies on homologous recombination (HR). The balance between these two mechanisms is essential since it defines the level of mutagenesis during lesion bypass, allowing genetic variability and adaptation to the environment, but also introduces the risk of generating genome instability. Here we report that the mere proximity of replication-blocking lesions that arise in Escherichia coli's genome during a genotoxic stress leads to a strong increase in the use of the error-prone TLS. We show that this increase is caused by the local inhibition of HR due to the overlapping of single-stranded DNA regions generated downstream of the lesions. This increase in TLS is independent of SOS activation, but its mutagenic effect is additive with the one of SOS. Hence, the combination of SOS induction and lesions proximity leads to a strong increase in TLS that becomes the main lesion tolerance pathway used by the cell during a genotoxic stress.