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J. Bacteriol., 11 1996, 6599-6607, Vol 178, No. 22
L Paul and JA Krzycki
The sequence and transcript of the genes encoding a recently discovered
coenzyme M methylase in Methanosarcina barkeri were analyzed. This 480- kDa
protein is composed of two subunits in equimolar concentrations which bind
one corrinoid cofactor per alphabeta dimer. The gene for the alphabeta
polypeptide, mtsA, is upstream of that encoding the beta polypeptide, mtsB.
The two genes are contiguous and overlap by several nucleotides. 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 upstream of mtsA. The 5' end of the mts
transcript was 19 nucleotides upstream of the translational start site of
mtsA and was positioned 25 bp from the center of the proximal BoxA
sequence. The transcript was most abundant in cells grown to the late log
phase on acetate but barely detectable in cells grown on methanol or
trimethylamine. The amino acid sequence of MtsB was homologous to the
cobalamin-binding fragment of methionine synthase from Escherichia coli and
possessed the signature residues involved in binding the corrinoid,
including a histidyl residue which ligates cobalt. The sequence of MtsA is
homologous to the "A" and "M" isozymes of methylcobamide:coenzyme M
methyltransferases (methyltransferase II), indicating that the alpha
polypeptide is a new member of the methyltransferase II family of coenzyme
M methylases. All 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 binding by proteins involved in methylotrophic methanogenesis are
discussed.
Copyright © 1996, American Society for Microbiology
Sequence and transcript analysis of a novel Methanosarcina barkeri methyltransferase II homolog and its associated corrinoid protein homologous to methionine synthase
Department of Microbiology, Ohio State University, Columbus 43210, USA.
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