A highly efficient and stable system for site-specific integration of genes and plasmids into the phage φLC3 attachment site ( attB) of the Lactococcus lactis chromosome

An integration vector system based on the site-specific integration apparatus of the temperate lactococcal bacteriophage φLC3 was developed. A 1.6-kb recombinogenic DNA cassette, containing the φLC3 integrase gene ( int) and the phage attachment site ( attP), mediated site-specific integration of a single marker-gene, as well as of a replication-thermosensitive (-ts) plasmid (pINT2), into the φLC3 attB site of Lactococcus lactis subsp. lactis LM0230 chromosome after introduction of the DNA into the cells by electroporation. Both the marker gene and the pINT2 plasmid were stably inserted as single copies in an orientation-specific and integrase-dependent manner, the pINT2-ts replicon being stably maintained at temperatures both permissive and non-permissive for plasmid-directed replication. Essentially all transformants obtained with the pINT2 plasmid appeared to be integrants, demonstrating the remarkably high efficiency of the system. This high efficiency rendered possible the detection of transformation-plus-integration events using DNA directly obtained from ligase reaction mixtures, thus avoiding initial subcloning in a non-lactococcal strain and subsequent cointegration of foreign replication functions into the chromosome of L. lactis. The above results, the observation that the φLC3 attB site appear to be conserved in L. lactis, and the fact that the int-attP cassette functions efficiently in a non-φLC3-host strain, show that the φLC3 site-specific integration apparatus provides an efficient and `food grade' tool for stable integration of genetic elements into the chromosome of L. lactis.