ENANTIOSELECTIVE UPTAKE AND DEGRADATION OF THE CHIRAL HERBICIDE DICHLORPROP[(RS)-2-(2,4-DICHLOROPHENOXY)PROPANOIC ACID] BY SPHINGOMONAS HERBICIDOVORANS MH

Sphingomonas herbicidovarans MH was able to completely degrade both en antiomers of the chiral herbicide dichlorprop [(RS)-2-(2,4-dichlorophe noxy) propanoic acid], with preferential degradation of the (S) enanti omer over the (R) enantiomer. These results are in agreement with the recently reported enantioselective degradation of mecoprop [(RS)-2-(4- chloro-2-methylphenoxy) propanoic acid] by this bacterium (C. Zipper, K. Nickel, W. Angst, and H.-P. E. Kohler, Appl. Environ. Microbiol. 62 :4318-4322, 1996). Uptake of (R)-dichlorprop, (S)-dichlorprop, and 2,4 -D (2,4-dichlorophenoxyacetic acid) was inducible. Initial uptake rate s of cells grown on the respective substrate showed substrate saturati on kinetics with apparent affinity constants (K-t) of 108, 93, and 117 mu M and maximal velocities (V-max) of 19, 10, and 21 nmol min(-1) mg of protein(-1) for (R)-dichlorprop, (S)-dichlorprop, and 2,4-D, respe ctively, Transport of (R)-dichlorprop, (S)-dichlorprop, and 2,4-D was completely inhibited by various uncouplers and by nigericin but was on ly marginally inhibited by valinomycin and by the ATPase inhibitor N,N '-dicyclohexylcarbodiimine. Experiments on the substrate specificity o f the putative transport systems revealed that (R)-dichlorprop uptake was inhibited by (R) mecoprop but not by (S)-mecoprop, (S)-dichlorprop , or 2,4-D. On the other hand, the (S)-dichlorprop transport was inhib ited by (S)-mecoprop hut not by (R)-mecoprop, (R)-dichlorprop, or 2,4- D. These results provide evidence that the first step in the degradati on of dichlorprop, mecoprop, and 2,4-D by S. herbicidovorans is active transport and that three inducible, proton gradient-driven uptake sys tems exist: one for (R)-dichlorprop and (R)-mecoprop, another for (S)- dichlorprop and (S)-mecoprop, and a third for 2,4-D.