Identification of the theta-type minimal replicon of theLactococcus lactis subsp.lactis CNRZ270 lactose protease plasmid pUCL22

The replication region of pUCL22, the lactose-protease plasmid ofLactococcus lactis subsp.lactis (Lc. lactis) CNRZ270, was isolated on an 18-kbBamHI fragment, by cloning into pUCB300, anEscherichia coli vector encoding Em^r, and selecting for Em^r inLc. lactis MG1614. Subcloning and deletion analysis localized the replication region, namedRep22, on a 2.3-kb fragment. Replicons based on this region followed a theta-type mechanism of replication inLc. lactis. An internal 1251-bpDraI fragment ofRep22 used as a probe hybridized with numerous plasmids in bacteria from the generaLactococcus, Lactobacillus, andLeuconostoc. 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.