CLUSTERED GENES ENCODING THE METHYLTRANSFERASES OF METHANOGENESIS FROM MONOMETHYLAMINE

Coenzyme M (CoM) is methylated during methanogenesis from monomethyami ne in a reaction catalyzed by three proteins. Using monomethylamine, a 52-kDa polypeptide termed monomethylamine methyltransferase (MMAMT) m ethylates the corrinoid cofactor bound to a second polypeptide, monome thylamine corrinoid protein (MMCP). Methylated MMCP then serves as a s ubstrate for MT2-A, which methylates CoM, The genes for these proteins are clustered on 6.8 kb of DNA in Methanosarcina barkeri. The gene en coding MMCP (mtmC) is located directly upstream of the gene encoding M MAMT (mtmB). The gene encoding MT2-A (MtbA) was found 1.1 kb upstream of mtmC, but no obvious open reading frame was found in the intergenic region between mtbA and mtmC. A single monocistronic transcript was f ound for mtbA that initiated 76 be, from the translational start. Sepa rate transcripts of 2.4 and 4.7 kb were detected, both of which carrie d mtmCB. The larger transcript also encoded mtmP, which is homologous to the APC family of cationic amine per permeases and may therefore en code a methylamine permease..4 single transcriptional start site was f ound. 447 bp upstream of the translational start of mtmC MtmC possesse s the corrinoid binding motif found in corrinoid proteins involved in dimethylsulfide- and methanol-dependent methanogenesis, as well as in methionine synthase. The open reading frame of mtmB was interrupted by a single in-frame, midframe, UAG codon which was also found its mtmB from M, barkeri NIII. A mechanism that circumvents UAG-directed termin ation of translation must operate during expression of mtmB in this me thanogen.