A DEFICIENCY IN ASPARTATE BIOSYNTHESIS IN LACTOCOCCUS-LACTIS SUBSP LACTIS C2 CAUSES SLOW MILK COAGULATION

A mutant of fast milk-coagulating (Fmc(+)) Lactococcus lactis subsp. l actis C2, designated L. lactis KB4, was identified. Although possessin g the known components essential for utilizing casein as a nitrogen so urce, which include functional proteinase (PrtP) activity and oligopep tide, di-and tripeptide, and amino acid transport systems, KB4 exhibit ed a slow milk coagulation (Fmc(-)) phenotype. When the amino acid req uirements oft. lactis C2 were compared with those of KB4 by use of a c hemically defined medium, it was found that KB4 was unable to grow in the absence of aspartic acid. This aspartic acid requirement could als o be met by aspartate-containing peptides, The addition of aspartic ac id to milk restored the Fmc(+) phenotype of KB4. KB4 was found to be d efective in pyruvate carboxylase and thus was deficient in the ability to form oxaloacetate and hence aspartic acid from pyruvate and carbon dioxide, The results suggest that when lactococci are propagated in m ilk, aspartate derived from casein is unable to meet fully the nutriti onal demands of the lactococci, and they become dependent upon asparta te biosynthesis.