Role of the fused corrinoid/methyl transfer protein CmtA during CO-dependent growth of methanosarcina acetivorans

The genome of Methanosarcina acetivorans encodes three homologs, initially annotated as hypothetical fused corrinoid/methyl transfer proteins, which are highly elevated in CO-grown cells versus cells grown with alternate substrates. Based only on phenotypic analyses of deletion mutants, it was previously concluded that the homologs are strictly dimethylsulfide:coenzymeM(CoM) methyltransferases not involved in the metabolism ofCO(E. Oelgeschlager and M. Rother, Mol. Microbiol. 72:1260-1272, 2009). The homolog encoded by MA4383 (here designated CmtA) was reexamined via biochemical characterization of the protein overproduced in Escherichia coli. Purified CmtA reconstituted with methylcob(III)alamin contained a molar ratio of cobalt to protein of 1.0 ± 0.2. The UV-visible spectrum was typical of methylated corrinoid-containing proteins, with absorbance maxima at 370 and 420 nm and a band of broad absorbance between 450 and 600 nm with maxima at 525, 490, and 550 nm. CmtA reconstituted with aquocobalamin showed methyl-tetrahydromethanopterin:CoM (CH₃-THMPT:HS-CoM) methyltransferase activity (0.31 μmol/min/mg) with apparent K_m values of 135 μM for CH₃-THMPT and 277 μM for HS-CoM. The ratio of CH₃-THMPT:HS-CoM methyltransferase activity in the soluble versus membrane cellular fractions was 15-fold greater in CO-grown versus methanolgrown cells. A mutant strain deleted for the CmtA gene showed lower growth rates and final yields when cultured with growthlimiting partial pressures of CO, demonstrating a role for CmtA during growth with this substrate. The results establish that CmtA is a soluble CH₃-THSPT:HS-CoM methyltransferase postulated to supplement the membrane-bound CH₃-THMPT:HS-CoM methyltransferase during CO-dependent growth of M. acetivorans. Thus, we propose that the name of the enzyme encoded by MA4384 be CmtA (for cytoplasmic methyltransferase).