Genetic and functional characterization of dpp genes encoding a dipeptide transport system in Lactococcus lactis

The genes encoding a binding-protein-dependent ABC transporter for dipeptides (Dpp) were identified in Lactococcus lactis subsp. cremoris MG1363. Two (dppA and dppP) of the six ORFs (dppAdpPBCDF) encode proteins that are homologous to peptide- and pheromone-binding proteins. The dppP gene contains a chain-terminating nonsense mutation and a frame-shift that may impair its function. The functionality of the dpp genes was proven by the construction of disruption mutants via homologous recombination. The expression of DppA and various other components of the proteolytic system was studied in synthetic and peptide-rich media and by using isogenic peptide-transport mutants that are defective in one or more systems (Opp, DtpT and/or Dpp). In peptide-rich medium, DppA was maximally expressed in mutants lacking Opp and DtpT. DppA expression also depended on the growth phase and was repressed by tri-leucine and tri-valine. The effect of tri-leucine on DppA expression was abolished when leucine was present in the medium. Importantly, the Dpp system also regulated the expression of other components of the proteolytic system. This regulation was achieved via the internalization of di-valine, which caused a 30-50% inhibition in the expression of the proteinase PrtP and the peptidases PepN and PepC. Similar to the regulation of DppA, the repressing effect was no longer observed when high concentrations of valine were present. The intricate regulation of the components of the proteolytic system by peptides and amino acids is discussed in the light of the new and published data.