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 which 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 the 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.