Molecular characterization of a theta replication plasmid and its use for development of a two-component food-grade cloning system for Lactococcus lactis.

pCD4, a small, highly stable theta-replicating lactococcal plasmid, was used to develop a food-grade cloning system. Sequence analysis revealed five open reading frames and two putative cis-acting regions. None appears to code for undesirable phenotypes with regard to food applications. Functional analysis of the replication module showed that only the cis-acting ori region and the repB gene coding for the replication initiator protein were needed for the stable replication and maintenance of pCD4 derivatives in Lactococcus lactis. A two-component food-grade cloning system was derived from the pCD4 replicon. The vector pVEC1, which carries the functional pCD4 replicon, is entirely made up of L. lactis DNA and has no selection marker. The companion pCOM1 is a repB-deficient pCD4 derivative that carries an erythromycin resistance gene as a dominant selection marker. The pCOM1 construct can only replicate in L. lactis if trans complemented by the RepB initiator provided by pVEC1. Since only the cotransformants that carry both pVEC1 and pCOM1 can survive on plates containing erythromycin, pCOM1 can be used transiently to select cells that have acquired pVEC1. Due to the intrinsic incompatibility between these plasmids, pCOM1 can be readily cured from the cells grown on an antibiotic-free medium after the selection step. The system was used to introduce a phage resistance mechanism into the laboratory strain MG1363 of L. lactis and two industrial strains. The introduction of the antiphage barrier did not alter the wild-type plasmid profile of the industrial strains. The phenotype was stable after 100 generations and conferred an effective resistance phenotype against phages of the 936 and c2 species.