Exploitation of a chromosomally integrated lactose operon for controlled gene expression in Lactococcus lactis

Lactococcus lactis MG5267 is a plasmid-free strain in which the lactose operon is integrated in the bacterial chromosome. The chromosomal lacG gene which encodes phospho-beta-galactosidase was inactivated by a double cross-over integration event. Unexpectedly, the resultant mutant was shown to retain a Lac-positive phenotype. The lysin gene from Listeria monocytogenes bacteriophage LM-4 was subsequently integrated into the chromosome of this strain such that expression of the heterologous gene was mediated by the lactose operon promoter. Expression of the lysin gene was shown to be regulated by growth on lactose. This represents an important strategy for the controlled and stabilised expression of biotechnologically useful genes in L. lactis.