Chloromethane : tetrahydrofolate methyl transfer by two proteins from Methylobacterium chloromethanicum strain CM4

The cmuA and cmuB genes are required for growth of Methylobacterium chlorom ethanicum strain CM4 with chloromethane as the sole carbon source. While Cm uB was previously shown to possess methylcobalamin:tetrahydrofolate methylt ransferase activity, sequence analysis indicated that CmuA represented a no vel and so far unique two-domain methyltransferase/corrinoid-binding protei n involved in methyl transfer from chloromethane to a corrin moiety. CmuA w as purified from wild-type M. chloromethanicum strain CM4 and characterized as a monomeric, cobalt-containing and zinc-containing enzyme of molecular mass 67 kDa with a bound vitamin B-12 cofactor. In combination, CmuA and Cm uB proteins catalyze the in vitro transfer of the methyl group of chloromet hane to tetrahydrofolate, thus affording a direct link between chloromethan e dehalogenation and core C1 metabolism of Methylobacterium. Chloromethane dehalogenase activity in vitro is limited by CmuB, as formation of methylte trahydrofolate from chloromethane displays apparent Michaelis-Menten kineti cs with respect to methylated CmuA, with an apparent K-m of 0.27 mum and a V-max of 0.45 U.mg(-1). This contrasts with sequence-related systems for me thyl transfer from methanogens, which involve methyltransferase and corrino id protein components in well-defined stoichiometric ratios.