Pleiotropic transcriptional repressor CodY senses the intracellular pool of branched-chain amino acids in Lactococcus lactis

Proteolysis is essential for supplying Lactococcus lactis with amino acids during growth in milk. Expression of the major components of the L. lactis proteolytic system, including the cell wall proteinase (PrtP), the oligopep tide transport system (Opp) and at least four intracellular peptidases (Pep O1, PepN, PepC, PepDA2), was shown previously to be controlled negatively b y a rich nitrogen source. The transcription of prtP, opp-pepO1, pepN and pe pC genes is regulated by dipeptides in the medium. Random insertion mutants derepressed for nitrogen control in the expression of the oligopeptide tra nsport system were isolated using an opp-lacZ fusion. A third of the mutant s were targeted in the same locus. The product of the inactivated gene shar ed 48% identity with CodY from Bacillus subtilis, a pleiotropic repressor o f the dipeptide permease operon (dpp) and several genes including genes inv olved in amino acid degradation and competence induction. The signal contro lling CodY-dependent repression was searched for by analysing the response of the opp-lux fusion to the addition of 67 dipeptides with different amino acid compositions. Full correlation was found between the dipeptide conten t in branched-chain amino acids (BCAA; isoleucine, leucine or valine) and t heir ability to mediate the repression of opp-pepO1 expression. The repress ive effect resulting from specific regulatory dipeptides was abolished in L . lactis mutants affected in terms of their transport or degradation into a mino acids, showing that the signal was dependent on the BCAA pool in the c ell. Lastly, the repression of opp-pepO1 expression was stronger in a mutan t unable to degrade BCAAs, underlining the central role of BCAAs as a signa l for CodY activity. This pattern of regulation suggests that, in L. lactis and possibly other Gram-positive bacteria, CodY is a pleiotropic repressor sensing nutritional supply as a function of the BCAA pool in the cell.