Combinational variation of restriction modification specificities in Lactococcus lactis

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Three genes coding for a type I R-M system related to the class C enzymes have been identified on the chromosome of Lactococcus lactis strain IL 1403. In addition, plasmids were found that encode only the HsdS subunit that directs R-M specificity. The presence of these plasmids in IL 1403 conferred a new R-M phenotype on the host, indicating that the plasmid-encoded HsdS is able to interact with the chromosomally encoded HsdR and HsdM subunits. Such combinational variation of type I R-M systems may facilitate the evolution of their specificity and thus reinforce bacterial resistance against invasive foreign unmethylated DNA.

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