SUMMARY: Different mechanisms have been proposed previously for the biodegradation of monomethyl sulphate (MMS) in sp. and sp. Sulphate liberation from MMS in sp. M3C was previously shown to be O-dependent, whereas in several spp. the initiating step has been considered hitherto to be hydrolytic and catalysed by methyl sulphatase. In the present study, and strains were compared for their ability to oxidize MMS and its potential metabolites in the oxygen electrode. MMS-grown sp. M3C and sp. MS223 oxidized MMS with consumption of 0.5 mol O per mol of substrate, but they were unable to oxidize methanol. By repeatedly challenging MMS-grown with MMS in the electrode chamber, all the O in the electrode became exhausted, at which point SO - liberation stopped although excess MMS was available. SO - release resumed immediately when O was re-admitted to the electrode chamber. Thus liberation of SO - from MMS in the oxygen electrode was dependent on the continuing availability of O. sp. MS223 therefore closely resembled sp. M3C in its obligatory requirement for O in MMS degradation. Unlike sp. M3C, sp. MS223 was able to grow on methanol and methanol-grown cells oxidized methanol (0.5 mol O per mol of substrate) but not MMS. Cyclopropanol, an inhibitor of methanol dehydrogenase, abolished oxidation of methanol by methanol-grown sp. MS223 but did not affect oxidation of MMS by MMS-grown cells. Thus sp. MS223 expresses enzymes for oxidation of methanol when needed for growth on this compound, but not when grown on MMS. These results are consistent with the absence of methanol from the pathway for biodegradation of MMS by sp. MS223 and suggest that in at least some spp. an oxidative mechanism initiates biodegradation.


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