Enantioselective regulation of lactate racemization by LarR in Lactobacillus plantarum

Abstract

Lactobacillus plantarum is a lactic acid bacterium that produces a racemic mixture of L- and D-lactate from sugar fermentation. The interconversion of lactate isomers is performed by a lactate racemase (Lar) that is transcriptionally controlled by the L-/Dlactate ratio and maximally induced in the presence of L-lactate. We previously reported that the Lar activity depends on the expression of two divergently oriented operons: (i) the larABCDE operon encodes the nickel-dependent lactate racemase (LarA), its maturases (LarBCE), and a lactic acid channel (LarD), and (ii) the larR(MN)QO operon encodes a transcriptional regulator (LarR) and a four-component ABC-type nickel transporter [Lar(MN), in which theMand N components are fused, LarQ, and LarO]. LarR is a novel regulator of the Crp-Fnr family (PrfA group). Here, the role of LarR was further characterized in vivo and in vitro. We show that LarR is a positive regulator that is absolutely required for the expression of Lar activity. Using gel retardation experiments, we demonstrate that LarR binds to a 16-bp palindromic sequence (Lar box motif) that is present in the larRlarA intergenic region. Mutations in the Lar box strongly affect LarR binding and completely abolish transcription from the larA promoter (P larA). Two half-Lar boxes located between the Lar box and the - 35 box of P larA promote LarR multimerization on DNA, and point mutations within one or both half-Lar boxes inhibit P larA induction by L-lactate. Gel retardation and footprinting experiments indicate that L-lactate has a positive effect on the binding and multimerization of LarR, while D-lactate antagonizes the positive effect of L-lactate. A possible mechanism of LarR regulation by lactate enantiomers is proposed.