Linear Plasmids and Prophages in Gram-Negative Bacteria

Abstract

Among the known linear plasmids in Gram-negative bacteria, there are three well-characterized phages (N15, PY54 and ϕKO2) whose prophages replicate as linear plasmids with covalently closed hairpin ends (telomeres). Close to the left telomere, a gene (tel) resides encoding the protelomerase. This tyrosine recombinase-like enzyme has cleaving–joining activity and is responsible for the generation of the telomeres by processing a palindromic telomere resolution site (telRL) located adjacent to the tel gene. The protelomerase exerts its activity as cleaving–joining enzyme in a concerted action. Functional analyses with the N15 protelomerase TelN demonstrated that two TelN molecules recognize telRL and bind to the target site. Within the centre of telRL (telO), the double-stranded DNA is cut by staggered cleavages six base pairs apart centred around the axis of dyad symmetry of the target site. TelN is transiently attached to the 3′ ends of the cleaving sites by covalent binding. Then the new phosphodiester bond with the complementary strand is generated in a transesterification step. Protelomerases are multifunctional enzymes. Besides telomere formation, TelN was shown to be implicated in linear plasmid replication and in the regulation of plasmid partitioning.