Cell extracts of Sphingomonas herbicidovorans MH grown on (R)-mecoprop contained an enzyme activity that selectively converted (R)-mecoprop to 4-chloro-2-methylphenol, whereas extracts of cells grown on (S)-meco-prop contained an enzyme activity selective for the S enantiomer. Both reactions were dependent on ␣-ketoglutarate and ferrous ions. Besides 4-chloro-2-methylphenol, pyruvate and succinate were detected as products of the reactions. Labeling experiments with 18 O 2 revealed that both enzyme activities catalyzed a dioxygenation reaction. One of the oxygen atoms of pyruvate and one of the oxygen atoms of succinate were derived from molecular oxygen. Analysis of cell extracts obtained from cells grown on different substrates by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that growth on (R)-mecoprop and (S)-mecoprop caused the appearance of prominent protein bands at 34 and 32 kDa, respectively. Both protein bands were present when cells grew on the racemic mixture. The results demonstrate that S. herbicidovorans initiated the degradation of each enantiomer of mecoprop by a specific ␣-ketoglutarate-dependent dioxygenase. By comparing conversion rates of various phenoxy herbicides, we confirmed that the two enzyme activities were distinct from that of TfdA, which catalyzes the first step in the degradation of 2,4-dichlorophenoxyacetic acid in Ralstonia eutropha JMP134.Substituted phenoxyalkanoic acid herbicides are widely used in weed control. 2,4-Dichlorophenoxyacetic acid (2,4-D) and the chiral compounds (R,S)-2-(4-chloro-2-methylphenoxy)propionic acid (mecoprop) and (R,S)-2-(2,4-dichlorophenoxy)propionic acid (dichlorprop) are members of this class of herbicides. The herbicidal activities of the last two compounds are exclusively associated with the R enantiomers. Today, the pure R enantiomers of mecoprop and of dichlorprop are commercially available as mecoprop-P and dichlorprop-P, respectively. Yet many commercial formulations contain the racemic mixtures and not the pure R enantiomers (27). In order to understand the environmental fate of the chiral compound mecoprop, it is important to study the degradation of the enantiomers independently, as has been recently emphasized (1,2,9,22,29).Studies concerning the fate and the persistence of phenoxyalkanoic acids in soil revealed that degradation of these herbicides is caused mainly by microorganisms (17, 23). The breakdown of 4-chloro-2-methylphenoxyacetic acid (MCPA) and 2,4-D starts with the removal of the alkanoic acid side chain to yield the corresponding phenol (17). Consequently, a great deal of research effort was put into unravelling the subsequent degradation of 2,4-dichlorophenol and 4-chloro-2-methylphenol, the metabolites of 2,4-D and MCPA, respectively (10). In soil, the microbial metabolism of mecoprop leads to the appearance of 4-chloro-2-methylphenol (24). This metabolite is also formed in a mixed bacterial culture with mecoprop as the sole source of carbon and energy (14). So far, only two pure bacterial strains have be...

Sphingomonas herbicidovorans MH (previously designated Flavobacterium sp. strain MH) was able to utilize the chiral herbicide (RS)-2-(4-chloro-2-methylphenoxy)propionic acid (mecoprop) as the sole carbon and energy source. When strain MH was offered racemic mecoprop as the growth substrate, it could degrade both the (R) and the (S) enantiomer to completion, as shown by biomass formation, substrate consumption, and stoichiometric chloride release. However, the (S) enantiomer disappeared much faster from the culture medium than the (R) enantiomer. These results suggest the involvement of specific enzymes for the degradation of each enantiomer. This view was substantiated by the fact that resting cells of strain MH grown on (S)-mecoprop were able to degrade the (S) but not the (R) enantiomer of mecoprop. Accordingly, resting cells of strain MH grown on (R)-mecoprop preferentially metabolized the (R) enantiomer. Nevertheless, such cells could transform (S)-mecoprop at low rates. Oxygen uptake rates with resting cells confirmed the above view, as oxygen consumption was strongly dependent on the growth substrate. Cells grown on (R)-mecoprop showed oxygen uptake rates more than two times higher upon incubation with the (R) than upon incubation with the (S) enantiomer and vice versa.

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