IDENTIFICATION OF THE THETA-TYPE MINIMAL REPLICON OF THE LACTOCOCCUS-LACTIS SUBSP LACTIS CNRZ270 LACTOSE PROTEASE PLASMID PUCL22

The replication region of pUCL22, the lactose-protease plasmid of Lactococcus lactis subsp. lactis (Lc. lactis) CNRZ270, was isolated on an 18-kb BamHI fragment, by cloning into pUCB300, an Escherichia coli vector encoding Em(r), and selecting for EM(r) in Lc. lactis MG1614. Subcloning and deletion analysis localized the replication region, named Rep22, on a 2.3-kb fragment. Replicons based on this region followed a theta-type mechanism of replication in Lc. lactis. An internal 1251-bp DraI fragment of Rep22 used as a probe hybridized with numerous plasmids in bacteria from the genera Lactococcus, Lactobacillus, and Leuconostoc. In some strains, two or three coresident plasmids hybridized with the probe in stringent conditions. It appears, therefore, that this family of theta-type replicons is widely distributed in lactic acid bacteria and contains several incompatibility groups.