THE PROTEOLYTIC SYSTEMS OF LACTIC-ACID BACTERIA

Proteolysis in dairy lactic acid bacteria has been studied in great de tail by genetic, biochemical and ultrastructural methods. From these s tudies the picture emerges that the proteolytic systems of lactococci and lactobacilli are remarkably similar in their components and mode o f action. The proteolytic system consists of an extracellularly locate d serine-proteinase, transport systems specific for di-tripeptides and oligopeptides (> 3 residues), and a multitude of intracellular peptid ases. This review describes the properties and regulation of individua l components as well as studies that have led to identification of the ir cellular localization. Targeted mutational techniques developed in recent years have made it possible to investigate the role of individu al and combinations of enzymes in vivo. Based on these results as well as in vitro studies of the enzymes and transporters, a model for the proteolytic pathway is proposed. The main features are: (i) proteinase s have a broad specificity and are capable of releasing a large number of different oligopeptides, of which a large fraction falls in the ra nge of 4 to 8 amino acid residues; (ii) oligopeptide transport is the main route for nitrogen entry into the cell; (iii) all peptidases are located intracellularly and concerted action of peptidases is required for complete degradation of accumulated peptides.