Investigating Oenococcus oeni citrate locus expression: Role of citR and impact on acid tolerance

A previous adaptive evolution experiment performed on the wine bacterium Oenococcus oeni revealed the beneficial effect of slowing down citrate metabolism on the acid tolerance of this bacterium. This finding prompted us to investigate the regulation pathways of the cit locus expression in this lactic acid bacterium, an investigation which has never been undertaken before. Mapping the cit locus, composed of a long mae-maeP-citCDEFXG cluster preceded by citR in a divergent direction, highlighted the expression of a citR messenger overlapping the coding sequence of mae, the first gene of the operon. Transcriptional analysis of the locus suggested that the transcript of maeP, encoding the citrate permease, could be stabilize by this antisense. Furthermore, genetic complementation has demonstrated the transcriptional regulatory function of CitR on the cit operon expression. This work contributes to a new understanding of the regulation of citrate metabolism in O. oeni which is involved in diacetyl production giving a buttery aroma to wine. It also provides new evidence for the involvement of citrate metabolism in the acid stress response of this bacterium, therefore in its ability to conduct malolactic fermentation in very acid wines.