Fig 5.

Postsegregational cell killing. Plasmid-harbored R-M gene complexes tend to propagate as selfish genetic elements to promote their own survival. The R-M system present in a cell expresses both REase and MTase: the REase restricts the foreign DNA, and the MTase protects the host genome against cleavage by the cognate REase. The postsegregational loss of the R-M gene complex results in the loss of methylation. The REase, owing to its higher level of stability, attacks the unmodified host genome, resulting in cell death (see “Selfish Genes”). The R-M gene complex thus propagates in the clonal population, resulting in the addiction of the host cell.