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

Bacillus subtilis yheL encodes a Na+/H+ antiporter, whereas its paralogue, yqkI, encodes a novel antiporter that achieves a simultaneous Na+/R+ and ma lolactate antiport. B. subtilis yufR, a control in some experiments, encode s a Na+/malate symporter. YqkI complemented a malate transport mutant of Es cherichia coli if Na+ and lactate were present. YheL conferred Na+ uptake c apacity on everted membrane vesicles from an antiporter-deficient E. coli m utant that was consistent with a secondary Na+/H+ antiport, but YqkI-depend ent Na+ uptake depended on intravesicular malate and extravesicular lactate . YqkI-dependent lactate uptake depended on intravesicular malate and extra vesicular Na+. YqkI mediated an electroneutral exchange, which is proposed to be a malic(-2)-2H(+) (or fully protonated malate)/Na+-lactate(-1) antipo rt, Because the composite YqkI-mediated exchanges could be driven by gradie nts of the malate-lactate pair, this transporter could play a role in growt h of B. subtilis on malate at low protonmotive force. A mutant with a disru ption of yqkI exhibited an abrupt arrest in the mid-logarithmic phase of gr owth on malate when low concentrations of protonophore were present. Thus g rowth of B. subtilis to high density on a putatively nonfermentative dicarb oxylic acid substrate depends on a malolactate exchange at suboptimal proto nmotive force.