The yiaK-S operon is required for aerobic growth on L-ascorbate in several Enterobacteriaceae. Here we present evidence that the yiaX2 gene belonging to the yiaK-S operon of Klebsiella pneumoniae 13882, which encodes a protein similar to the putative transporters classified as the major facilitator superfamily, is involved in the uptake of L-ascorbate. Concentration kinetic analysis yielded an apparent K(m) of YiaX2 for L-ascorbate of 161.38 +/-8.28 micromol x L(-1) and a Vmax of 3.81 +/- 0.60 nmol x mg(-1) x min(-1). This carrier uses the energy from electrochemical gradients, since it was inhibited by carbonyl cyanide m-chlorophenylhydrazone, a hydrophobic proton conductor that dissipates proton motive force.