Unbalanced Restriction Impairs SOS-induced DNA Repair Effects

The contribution of a type Ⅱ restriction-modification system (R-M system) to genome integrity and cell viability was investigated. We established experimental conditions that enabled the achievement of hemimethylated and unmethylated states for the specific bases of the recognition sequences of the host's DNA. To achieve this, we constructed the MboⅡ R-M system containing only one (i.e., M2.MboⅡ) out of two functional MboⅡ methyltransferases found in Moraxella bovis. Using the incomplete R-M system, we were able to perturb the balance between methylation and restriction in an inducible manner. We demonstrate that upon the SOS-induced DNA repair in mitomycin C treated cells, restriction significantly reduces cell viability. Similar results for the well-studied wild-type EcoRI R-M system, expressed constitutively in Escherichia coli, were obtained. Our data provide further insights into the benefits and disadvantages of maintaining of a type Ⅱ R-M system, highlighting its impact on host cell fitness.