AN ATP-DEPENDENT L-CARNITINE TRANSPORTER IN LISTERIA-MONOCYTOGENES SCOTT-A IS INVOLVED IN OSMOPROTECTION

Listeria monocytogenes is a gram-positive, psychrotrophic, food-borne pathogen which is able to grow in osmotically stressful environments. Carnitine (beta-hydroxy-L-tau-N-trimethyl aminobutyrate) can contribute significantly to growth of L. monocytogenes at high osmolarity (R. R Beumer, M. C. te Giffel, L. J. Cox, F. M. Rombouts, and T. Abee, Appl, Environ, Microbiol. 60:1359-1363, 1994). Transport of L-[N-methyl-C-14] carnitine in L. monocytogenes was shown to be energy dependent. Analysis of cell extracts revealed that L-carnitine was not further metabolized, which supplies evidence for its role as an osmoprotectant in L. monocytogenes. Uptake of L-carnitine proceeds in the absence of a proton motive force and is strongly inhibited in the presence of the phosphate analogs vanadate and arsenate. The L-carnitine permease is therefore most likely driven by ATP. Kinetic analysis of L-carnitine transport in glucose-energized cells revealed the presence of a high-affinity uptake system with a K-m of 10 mu M and a maximum rate of transport (V-max) of 48 nmol min(-1) mg of protein(-1), L-[C-14]carnitine transport in L. monocytogenes is significantly inhibited by a 10-fold excess of unlabelled L-carnitine, acetylcarnitine, and tau-butyrobetaine, whereas L-proline and betaine display, even at a 100-fold excess, only a weak inhibitory effect, In conclusion, an ATP-dependent L-carnitine transport system in L. monocytogenes is described, and its possible roles in cold adaptation and intracellular growth in mammalian cells are discussed.