Bacillus subtilis YqkI is a novel malic/Na+-lactate antiporter that enhances growth on malate at low protonmotive force.

Abstract Bacillus subtilis yheL encodes a Na+/H+ antiporter, whereas its paralogue,yqkI, encodes a novel antiporter that achieves a simultaneous Na+/H+ and malolactate antiport. B. subtilis yufR, a control in some experiments, encodes a Na+/malate symporter. YqkI complemented a malate transport mutant of Escherichia coliif Na+ and lactate were present. YheL conferred Na+ uptake capacity on everted membrane vesicles from an antiporter-deficient E. coli mutant that was consistent with a secondary Na+/H+ antiport, but YqkI-dependent Na+ uptake depended on intravesicular malate and extravesicular lactate. YqkI-dependent lactate uptake depended on intravesicular malate and extravesicular Na+. YqkI mediated an electroneutral exchange, which is proposed to be a malic−2-2H+ (or fully protonated malate)/Na+-lactate−1 antiport. Because the composite YqkI-mediated exchanges could be driven by gradients of the malate-lactate pair, this transporter could play a role in growth ofB. subtilis on malate at low protonmotive force. A mutant with a disruption of yqkI exhibited an abrupt arrest in the mid-logarithmic phase of growth on malate when low concentrations of protonophore were present. Thus growth of B. subtilis to high density on a putatively nonfermentative dicarboxylic acid substrate depends on a malolactate exchange at suboptimal protonmotive force.