METHANOL-COENZYME-M METHYLTRANSFERASE FROM METHANOSARCINA-BARKERI - PURIFICATION, PROPERTIES AND ENCODING GENES OF THE CORRINOID PROTEIN MT1

In Methanosarcina barkeri, methanogenesis from methanol is initiated b y the formation of methyl-coenzyme M from methanol and coenzyme M. Thi s methyl transfer reaction is catalyzed by two enzymes, designated MT1 and MT2. Transferase MT1 is a corrinoid protein. The purification, ca talytic properties and encoding genes of MT2 (MtaA) have been describe d previously [Harms, U. and Thauer, R. K. (1996) Eur. J. Biochem. 235, 653-659]. We report here on the corresponding analysis of MT1. The co rrinoid protein MT1 was purified to apparent homogeneity and showed a specific activity of 750 mu mol min(-1) mg(-1). The enzyme catalyzed t he methylation of its bound corrinoid in the cob(I)amide oxidation sta te by methanol. In addition to this automethylation, the purified enzy me was found to catalyze the methylation of free cob(I)alamin to methy lcob(III)alamin. It was composed of two different subunits designated MtaB and MtaC, with apparent molecular masses of 49 kDa and 24 kDa, re spectively. The subunit MtaC was shown to harbour the corrinoid prosth etic group. The genes mtaB and mtaC were cloned and sequenced, They we re found to be juxtapositioned and to form a transcription unit mtaCB. The corrinoid-harbouring subunit MtaC exhibits 35% sequence similarit y to the cobalamin-binding domain of methionine synthase from Escheric hia coli.