SEQUENCE AND TRANSCRIPT ANALYSIS OF A NOVEL METHANOSARCINA-BARKERI METHYLTRANSFERASE-II HOMOLOG AND ITS ASSOCIATED CORRINOID PROTEIN HOMOLOGOUS TO METHIONINE SYNTHASE

The sequence and transcript of the genes encoding a recently discovere d coenzyme M methylase in Methanosarcina barkeri were analyzed. This 4 80-kDa protein is composed of two subunits in equimolar concentrations which bind one corrinoid cofactor per alpha beta dimer. The gene for the alpha polypeptide, mts, is upstream of that encoding the beta poly peptide, mtsB. The two genes are contiguous and overlap by several nuc leotides. A 1.9-kb mRNA species which reacted with probes specific For either mtsA or mtsB was detected, Three possible methanogen consensus BoxA sequences as well as two sets of direct repeats were found upstr eam of mtsA. The 5' end of the mts transcript was 19 nucleotides upstr eam of the translational start site of mtsA and was positioned 25 bp f rom the center of the proximal BoxA sequence, The transcript was most abundant in cells grown to the late log phase on acetate but barely de tectable in cells grown on methanol or trimethylamine. The amino acid sequence of MtsB was homologous to the cobalamin-binding fragment of m ethionine synthase from Escherichia coli and possessed the signature r esidues involved in binding the corrinoid, including a histidyl residu e which Ligates cobalt, The sequence of MtsA is homologous to the ''A' ' end ''M'' isozymes of methylcobamide:coenzyme hi methyltransferases (methyltransferase II), indicating that the alpha polypeptide is a new member of the methyltransferase II family of coenzyme M methylases. A ll three methyltransferase II homolog sequences could be aligned with the sequences of uroporphyrinogen decarboxylase from various sources, The implications of these homologies for the mechanism of corrinoid bi nding by proteins involved in methylotrophic methanogenesis are discus sed.