Evasion of type I and type II DNA restriction systems by Incl1 plasmid Collb‐P9 during transfer by bacterial conjugation

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Abstract
Transmission of unmodified plasmid Collb-P9 by bacterial conjugation is markedly resistant to restriction compared with transfer by transformation. One process allowing evasion of type I and II restriction systems involves conjugative transfer of multiple copies of the plasmid. A more specialized evasion mechanism requires the Ard (alleviation of restriction of DNA) system encoded by Collb. The ard gene is transferred early in conjugation and specifically alleviates DNA restriction by all known families of type I enzyme, including Eco K. Collb has no effect on EcoK modification but this activity is impaired by multicopy recombinant plasmids supporting overexpression of ard. Genetic evidence shows that Ard protects Collb from Eco K restriction following conjugative transfer and that this protection requires expression of the gene on the immigrant plasmid. It is proposed that carriage of ard facilitates transfer of Collb between its natural enterobacterial hosts and that the route of DNA entry is important to the restriction-evasion mechanism.