Genetic and Physiological Probing of Cytoplasmic Bypasses for the Energy-Converting Methyltransferase Mtr in Methanosarcina acetivorans.

Methanogenesis is a unique energy metabolism carried out by members of the domain . Unlike most other methanogens, which reduce CO to methane with hydrogen as the electron donor, Methanosarcina acetivorans is able to grow on methylated compounds, on acetate, and on carbon monoxide (CO). These substrates are metabolized via distinct yet overlapping pathways. For the use of any single methanogenic substrate, the membrane-integral, energy-converting -methyl-tetrahydrosarcinapterin (HSPT):coenzyme M (HS-CoM) methyltransferase (Mtr) is required. It was proposed that can bypass the methyl transfer catalyzed by Mtr via cytoplasmic activities. To address this issue, conversion of different energy substrates by an deletion mutant was analyzed. No significant methyl transfer from HSPT to HS-CoM could be detected with CO as the electron donor. In contrast, formation of methane and CO in the presence of methanol or trimethylamine was indicative of an Mtr bypass in the oxidative direction. As methane thiol and dimethyl sulfide were transiently produced during methylotrophic methanogenesis in the mutant, involvement in this process of methyl sulfide-dependent methyltransferases (Mts) was analyzed in a strain lacking both the Mts system and Mtr. It could be unequivocally demonstrated that the Mts system is not involved in bypassing Mtr, thereby ruling out previous proposals. Conversion of [C]methanol indicated that in the absence of Mtr provides the reducing equivalents for methyl-S-CoM reduction to methane by oxidizing (an) intracellular compound(s) to CO rather than disproportioning the source of methyl groups. Thus, no Mtr bypass appears to exist in . Methanogenic archaea possess only a limited number of chemiosmotic coupling sites in their respiratory chains. Among them, -methyl-HSPT:HS-CoM methyltransferase (Mtr) is the most widely distributed. Previous observations led to the conclusion that Methanosarcina acetivorans is able to bypass this reaction via methyl sulfide-dependent methyltransferases (Mts). However, strains lacking Mtr are not able to produce methane from CO. Also, these strains are unable to oxidize methylated substrates to CO, in contrast to observations in the close relative Methanosarcina barkeri. The results also highlight the sole function of the Mts system in methyl sulfide metabolism. Thus, no Mtr bypass appears to exist in .