COMPARISON OF PATHWAYS FOR BIODEGRADATION OF MONOMETHYL SULFATE IN AGROBACTERIUM AND HYPHOMICROBIUM SPECIES

Different mechanisms have been proposed previously for the biodegradat ion of monomethyl sulphate (MMS) in Agrobacterium sp. and Hyphomicrobi um sp. Sulphate liberation from MMS in Agrobacterium sp. M3C was previ ously shown to be O-2-dependent, whereas in several Hyphomicrobium spp . the initiating step has been considered hitherto to be hydrolytic an d catalysed by methyl sulphatase. In the present study, Agrobacterium and Hyphomicrobium strains were compared for their ability to oxidize MMS and its potential metabolites in the oxygen electrode. MMS-grown A grobacterium sp. M3C and Hyphomicrobium sp. MS223 oxidized MMS with co nsumption of 0.5 mol O-2 per mol of substrate, but they were unable to oxidize methanol. By repeatedly challenging MMS-grown Hyphromicrobium with MMS in the electrode chamber, all the O-2 in the electrode becam e exhausted, at which point SO42- liberation stopped although excess M MS was available. SO42- release resumed immediately when O-2 was re-ad mitted to the electrode chamber. Thus liberation of SO42- from MMS in the oxygen electrode was dependent on the continuing availability of O ,. Hyphomicrobium sp. MS223 therefore closely resembled Agrobacterium sp. M3C in its obligatory requirement for O-2 in MMS degradation. Unli ke Agrobacterium sp. M3C, Hyphomicrobium sp. MS223 was able to grow on methanol and methanol-grown cells oxidized methanol (0.5 mol O-2 per mol of substrate) but not MMS. Cyclopropanol, an inhibitor of methanol dehydrogenase, abolished oxidation of methanol by methanol-grown Hyph omicrobium sp. MS223 but did not affect oxidation of MMS by MMS-grown cells. Thus Hyphomicrobium sp. MS223 expresses enzymes for oxidation o f methanol when needed for growth on this compound, but not when grown on MMS. These results are consistent with the absence of methanol fro m the pathway for biodegradation of MMS by Hyphomicrobium sp. MS223 an d suggest that in at least some Hyphomicrobium spp. an oxidative mecha nism initiates biodegradation.