Optimizing DNA restriction for host defense: strategies in Type II restriction-modification system beyond a single mechanism

The effective functioning of bacterial restriction-modification systems (R-Ms) hinges on their ability to degrade foreign DNA, which require precise optimization of gene expression tailored to the host cell’s genetic environment and regulatory landscape. The goal of this study was to determine how gene copy number influences the performance of Type II R-Ms and to explain why some systems fail under overexpression. Here, we examined three R-M systems isolated from natural plasmids of Enterobacteriaceae —EcoRI, EcoVIII, and PvuII. All three systems were ineffective from single-copy constructs, whereas elevating gene copy number markedly enhanced DNA restriction. Strikingly, EcoRI exhibited a nonlinear response, with intermediate gene dosage yielding maximal restriction and further increases sharply reducing its efficiency. In contrast, EcoVIII and PvuII showed a monotonic increase that approached saturation at higher gene copy numbers. Under the conditions tested in this study, we reveal the new aspects in biology of some Type II R-M systems, that excessive expression (even 5-fold higher than optimal) or above the host-optimized range, can reduce degradation of invading bacteriophage DNA. To interpret these trends, we developed a minimal stochastic–mechanistic model linking gene copy number to cell-to-cell abundances of DNA methyltransferase and restriction endonuclease, explicitly incorporating few-molecule noise at low expression and the nonlinear formation of active endonuclease dimers (with EcoRI-specific dimer sequestration at high expression). By treating methylation and cleavage as competing events at multiple recognition sites on the invading phage genome, the model yields model-calculated phage-escape probabilities that recapitulate the observed copy-number dependence of restriction, including EcoVIII’s saturating response and EcoRI’s peak-and-decline behavior under overexpression. This novel observation shows that the relationship between gene dosage and restriction efficiency is system-specific rather than universal. Taken together, these findings reveal that the balance between restriction endonuclease and DNA methyltransferase activities is exquisitely sensitive to transcriptional signals both upstream and downstream of the R-Ms operon. Our work highlights the critical need to fine-tune R-M gene expression for robust bacterial defense, deepening insights into prokaryotic immunity and guiding biotechnological engineering of these systems.