GENETIC EVIDENCE THAT LACTOCOCCUS LACTIS JCM7638 PRODUCES A MUTATED FORM OF NISIN

Abstract

Nisin is a 34 amino acid peptide antibiotic having a molecular weight of about 3, 340, produced and exported into culture medium by some strains of Lactococcus lactis. It is used as a food preservative for its high potency against certain gram-positive bacteria. The structural genes of nisin have been found and sequenced from several nisin-producing strains, which revealed the existence of 23 residue leader peptide and confirmed posttranslational processing of this substance. Recently a newly found strain of L. lactis, JCM7638 (previous IO-1), was reported to secrete a new peptide antibiotic having a molecular weight of about 2, 700 and a spectrum of action similar to nisin. Several chemical analyses showed that it is quite similar to nisin. In this paper, we have ascertained the resistance of JCM7638 to nisin, showed the existence of the gene which is highly homologous to the structural gene of nisin in a nisin-producing strain, L. lactis NCDO497, and cloned a 1.7kb Sau 3AI fragment encoding this gene. Sequence analysis proved that the deduced product differs from nisin by a single amino acid substitution (His₂₇→Asn₂₇), which strongly suggested that JCM7638 secretes degraded nisin and that His₂₇ is not essential to its antibiotic activity.