Lactococcin 972: A homodimeric lactococcal bacteriocin whose primary target is not the plasma membrane

Lactococcus lactis subsp. lactis IPLA 972 was shown to produce a bacteriocin which had a bactericidal effect on sensitive lactococci. Production of lactococcin 972 reached a maximum during the late-exponential phase of growth. The bacteriocinogenic activity was heat-sensitive, active in the pH range 4.0-9.0 and showed low susceptibility to proteases. Purification of the bacteriocin rendered a single polypeptide of 7.5 kDa (monomer) as shown by SDS-PAGE. Gels overlaid with a lawn of sensitive bacteria showed inhibitory activity at a point corresponding to 15 kDa. Changes in the electrophoretic conditions allowed the detection of a band at a position corresponding to that expected for a hypothetical dimer. Sequencing of the NH2-terminal end of lactococcin 972 revealed the sequence NH2-EGTWQHGYGV, which is not related to any other bacteriocin sequence present in the databases. Finally, lactococcin 972 did not induce the efflux of compounds previously incorporated into the cytoplasm of sensitive cultures nor did it inhibit macromolecular synthesis, suggesting that, in contrast to other bacteriocins, its primary target is not the plasma membrane.