Ps. Grigoriev et Mb. Lobocka, Determinants of segregational stability of the linear plasmid-prophage N15of Escherichia coli, MOL MICROB, 42(2), 2001, pp. 355-368
N15 is a bacteriophage of Escherichia coli that resembles lambda, but, unli
ke lambda, it lysogenizes as a linear plasmid. We show that stable maintena
nce of this unusual plasmid-prophage depends on the parA and parB genes, re
latives of the partition genes of F and P1 plasmids. ParB of N15, like its
F- and Pl-encoded homologues, destabilizes plasmids carrying its target cen
tromere, when present in excess. Within the genome of N15, we identified fo
ur unlinked, palindromic sequences that can promote the ParB-mediated desta
bilization of a moderate-copy vector in cis. They are distant from the parA
B operon, unlike the centromeric sites of F and P1. Each of these palindrom
es could interact in vivo and in vitro with ParB. Each, when cloned separat
ely, had properties characteristic of centromeric sites: exerted incompatib
ility against the N15 prophage and mini-N15 plasmids, and stabilized a mini
-P1 plasmid depleted of its own partition genes when ParA and ParB of N15 w
ere supplied. A pair of sites was more effective than a single site. Two of
the centromeric sites are located in the proximity of promoters of phage g
enes, suggesting that, in addition to their function in partitioning of N15
prophage, they may control expression of N15 lytic functions.