DNA lesions proximity modulates damage tolerance pathways

The genome of all organisms is constantly threatened by numerous agents that cause DNA damages. 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 (DA) that relies on homologous recombination. 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 introducing the risk of generating genome instability. Here we report that the mere proximity of replication-blocking lesions located in opposite strands of the bacterial genome leads to a strong increase in the use of the error-prone TLS. This increase is caused by the local inhibition of homologous recombination due to the overlapping of single-stranded DNA regions generated downstream the lesions, and appears as a way for cells to respond to genotoxic stress by favoring the potentially mutagenic translesion synthesis pathway. We show that this response is independent of SOS activation, but that its mutagenic effect is additive with the one of SOS induction. Hence, the combination of SOS induction and lesions proximity leads to strong increase in the use of TLS that becomes the main lesion tolerance pathway used by the cell.