Cloning and analysis of the pepV dipeptidase gene of Lactococcus lactis MG1363.

The gene pepV, encoding a dipeptidase from Lactococcus lactis subsp. cremoris MG1363, was identified in a genomic library in pUC19 in a peptidase-deficient Escherichia coli strain and subsequently sequenced. PepV of L. lactis is enzymatically active in E. coli and hydrolyzes a broad range of dipeptides but no tri-, tetra-, or larger oligopeptides. Northern (RNA) and primer extension analyses indicate that pepV is a monocistronic transcriptional unit starting 24 bases upstream of the AUG translational start codon. The dipeptidase of L. lactis was shown to be similar to the dipeptidase encoded by pepV of L. delbrueckii subsp. lactis, with 46% identity in the deduced amino acid sequences. A PepV-negative mutant of L. lactis was constructed by single-crossover recombination. Growth of the mutant strain in milk was significantly slower than that of the wild type, but the strains ultimately reached the same final cell densities.

Multiple-peptidase mutants of Lactococcus lactis are severely impaired in their ability to grow in milk.

To examine the contribution of peptidases to the growth of lactococcus lactis in milk, 16 single- and multiple-deletion mutants were constructed. In successive rounds of chromosomal gene replacement mutagenesis, up to all five of the following peptidase genes were inactivated (fivefold mutant): pepX, pepO, pepT, pepC, and pepN. Multiple mutations led to slower growth rates in milk, the general trend being that growth rates decreased when more peptidases were inactivated. The fivefold mutant grew more than 10 times more slowly in milk than the wild-type strain. In one of the fourfold mutants and in the fivefold mutant, the intracellular pools of amino acids were lower than those of the wild type, whereas peptides had accumulated inside the cell. No significant differences in the activities of the cell envelope-associated proteinase and of the oligopeptide transport system were observed. Also, the expression of the peptidases still present in the various mutants was not detectably affected. Thus, the lower growth rates can directly be attributed to the inability of the mutants to degrade casein-derived peptides. These results supply the first direct evidence for the functioning of lactococcal peptidases in the degradation of milk proteins. Furthermore, the study provides critical information about the relative importance of the peptidases for growth in milk, the order of events in the proteolytic pathway, and the regulation of its individual components.