In the proteolytic pathway of Lactococcus lactis, milk proteins (caseins) are hydrolyzed extracellularly to oligopeptides by the proteinase (PrtP). The fate of these peptides, i.e. extracellular hydrolysis followed by amino acid uptake or transport followed by intracellular hydrolysis, has been addressed. Mutants have been constructed that lack a functional di-tripeptide transport system (DtpT) and/or oligopeptide transport system (Opp) but do express the P-type proteinase (specific for hydrolysis of - and to a lesser extent -casein). The wild type strain and the DtpT mutant accumulate all casein-derived amino acids in the presence of -casein as protein substrate and glucose as a source of metabolic energy. The amino acids are not accumulated significantly inside the cells by the Opp and DtpT Opp mutants. When cells are incubated with a mixture of amino acids mimicking the composition of -casein, the amino acids are taken up to the same extent in all four strains. Analysis of the extracellular peptide fraction, formed by the action of PrtP on -casein, indicates that distinct peptides disappear only when the cells express an active Opp system. These and other experiments indicate that (i) oligopeptide transport is essential for the accumulation of all -casein-derived amino acids, (ii) the activity of the Opp system is sufficiently high to support high growth rates on -casein provided leucine and histidine are present as free amino acids, and (iii) extracellular peptidase activity is not present in L. lactis.

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