The citrate transport system of Lactococcus lactis subsp. lactis biovar diacetylactis is induced by acid stress.

Citrate transport in Lactococcus lactis subsp. lactis biovar diacetylactis is catalyzed by citrate permease P (CitP), which is encoded by the plasmidic citP gene. We have shown previously that citP is included in the citQRP operon, which is mainly transcribed from the P1 promoter in L. lactis subsp. lactis biovar diacetylactis. furthermore, transcription of citQRP and citrate transport are not induced by the presence of citrate in the growth medium. In this work, we analyzed the influence of the extracellular pH on the expression of citP. The citrate transport system is induced by natural acidification of the medium during cell growth and by a shift to media buffered at acidic pHs. This inducible response to acid stress takes place at the transcriptional level and seems to be due to increased utilization of the P1 promoter. Increased transcription correlates with increased synthesis of CitP and results in higher citrate transport activity catalyzed by the cells. Finally, this acid stress response seems to provide L. lactis subsp. lactis biovar diacetylactis with a selective advantage resulting from cometabolism of glucose and citrate at low pHs.