The proteolytic system of lactococci that allows degradation of caseins and
proteins of milk is complex. Milk proteins contain all amino acids necessa
ry for growth of lactic acid bacteria. The proteolytic system consists of a
n extracellularly located proteinase, transport systems for di-tripeptides
and oligopeptides and a multitude of intracellular peptidases. Expression o
f 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 di
peptides containing one of the 3 branched amino acids (isoleucine, leucine
and valine). Repression of gene transcription required that regulatory pept
ides are translocated into the cell and degraded in amino acids. Cell facto
rs involved in this regulation were identified in derepressed mutants obtai
ned by random mutagenesis by transposition. DtpT, a di-tripeptides transpor
ter 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 ge
nes of the proteolytic system belong to a same regulon since their expressi
on is repressed by CodY regulator depending on intracellular concentration
of branched amino acids or derivative products of them.