Network of regulation of gene transcription of the proteolytic system of Lactococcus lactis

The proteolytic system of lactococci that allows degradation of caseins and proteins of milk is complex. Milk proteins contain all amino acids necessary for growth of lactic acid bacteria. The proteolytic system consists of an extracellularly located proteinase, transport systems for di-tripeptides and oligopeptides and a multitude of intracellular peptidases. Expression of 13 genes was followed by transcriptional fusions in presence of different peptide sources. Transcription of 6 genes is repressed in media containing peptides and that of 4 genes (pepN, pepC, prtP and opp-pepO1 operon) by dipeptides containing one of the 3 branched amino acids (isoleucine, leucine and valine). Repression of gene transcription required that regulatory peptides are translocated into the cell and degraded in amino acids. Cell factors involved in this regulation were identified in derepressed mutants obtained by random mutagenesis by transposition. DtpT, a di-tripeptides transporter and CodY, homologous of the Bacillus subtilis pleiotropic regulator of transcription were the most frequently inactivated proteins. pepC, pepN and opp-pepO1 transcription is not repressed in codY and dtpT mutant. These genes of the proteolytic system belong to a same regulon since their expression is repressed by CodY regulator depending on intracellular concentration of branched amino acids or derivative products of them.