TRANSPORT OF BETA-CASEIN-DERIVED PEPTIDES BY THE OLIGOPEPTIDE TRANSPORT-SYSTEM IS A CRUCIAL STEP IN THE PROTEOLYTIC PATHWAY OF LACTOCOCCUS-LACTIS

In the proteolytic pathway of Lactococcus lactis, milk proteins (casei ns) are hydrolyzed extracellularly to oligopeptides by the proteinase (PrtP). The fate of these peptides, i.e. extracellular hydrolysis foll owed by amino acid uptake or transport followed by intracellular hydro lysis, has been addressed. Mutants have been constructed that lack a f unctional di-tripeptide transport system (DtpT) and/or oligopeptide tr ansport system (Opp) but do express the P-1-type proteinase (specific for hydrolysis of beta- and to a lesser extent kappa-casein). The wild type strain and the DtpT(-) mutant accumulate all beta-casein-derived amino acids in the presence of beta-casein as protein substrate and g lucose as a source of metabolic energy. The amino acids are not accumu lated significantly inside the cells by the Opp(-) and DtpT(-) Opp(-) mutants. When cells are incubated with a mixture of amino acids mimick ing the composition of beta-casein, the amino acids are taken up to th e same extent in all four strains. Analysis of the extracellular pepti de fraction, formed by the action of PrtP on beta-casein, indicates th at distinct peptides disappear only when the cells express an active O pp system. These and other experiments indicate that (i) oligopeptide transport is essential for the accumulation of all beta-casein-derived amino acids, (ii) the activity of the Opp system is sufficiently high to support high growth rates on beta-casein provided leucine and hist idine are present as free amino acids, and (iii) extracellular peptida se activity is not present in L. lactis.