Molecular characterization of the genes involved in the secretion and immunity of lactococcin Q, a two-peptide bacteriocin produced by Lactococcus lactis QU 4

Lactococcin Q is a two-peptide (Q alpha and Q beta) bacteriocin produced by Lactococcus lactis QU 4, which exhibits specific antimicrobial activity against L. lactis strains. The lactococcin gene cluster (approximately 4.5 kb) was sequenced and found to include genes encoding lactococcin Q immunity (laqC), an ATP-binding cassette transporter (laqD) and a transport accessory protein (laqE), downstream of the lactococcin Q structural genes (laqA and laqB). In addition, the gene cluster showed high sequence identity with that of a lactococcin homologue bacteriocin, lactococcin G. Heterologous expression studies showed that LaqD was responsible for lactococcin Q secretion in a manner dependent on LaqE expression, and that LaqC conferred self-immunity to lactococcin Q and cross-immunity to lactococcin G. Amino acid alignment of both lactococcin transporters revealed that LaqD contains an insertion (160-168 residues) that is essential for lactococcin Q secretion, as L. lactis cells that expressed LaqD(Delta 160-168) were devoid of this function. Additional experiments demonstrated that the LaqD(Delta 160-168) mutant was, however, able to secrete lactococcin G, suggesting that the insertion is necessary only for the lactococcin Q secretion by LaqD. This report demonstrates the biosynthetic mechanism of lactococcin Q/G-type bacteriocins and the complennentarity of the genes responsible for the secretion of lactococcins Q and G.