REGULATION OF PROTEOLYTIC-ENZYME ACTIVITY IN LACTOCOCCUS-LACTIS

Two different Lactococcus lactis host strains, L. lactis subsp. lactis MG1363 and L. lactis subsp. cremoris SK1128, both containing plasmid pNZ521, which encodes the extracellular serine proteinase (PrtP) from strain SK110, were used to study the medium and growth-rate-dependent activity of three different enzymes involved in the proteolytic system of lactococci. The activity Levels of PrtP and both the intracellular aminopeptidase PepN and the X-prolyl-dipeptidyl aminopeptidase PepXP were studied during batch and continuous cultivation, In both strains, the PrtP activity level was regulated by the peptide content of the m edium, The highest activity level was found during growth in milk, and the Lowest level was found during growth in tile peptide-rich laborat ory medium M17. Regulation of the intracellular peptidase activity app eared to be a strain-dependent phenomenon, In cells of strain MG1363, the activity Levels of PepN acid PepXP were regulated in a similar way to that observed for PrtP. In cells of strain SK1128, the levels of b oth peptidases were not significantly influenced by the peptide conten t of the medium, The presence of specific concentrations of the dipept ide prolylleucine could mimic the low activity levels of the regulated proteolytic enzymes, even to the activity level found on M17 medium. The effect of the presence of the dipeptide prolylleucine in the mediu m on the activity level of the regulated proteolytic enzymes was confi rmed at fired growth rates in chemostat cultures.