Methyl-coenzyme M reductase (MCR) catalyses the methane-forming step in the energy metabolism of methanogenic Archaea. It brings about the reduction of methyl-coenzyme M (CH,-S-CoM) by 7-mercaptoheptanoylthreonine phosphate (H-S-HTP). Methanobacterium thermoautotrophicunz contains two isoenzymes of MCR, designated MCR I and MCR 11, which are expressed differentially under different conditions of growth. These two isoenzymes have been separated, purified and their catalytic and spectroscopic properties determined. Initial-velocity measurements of the twosubstrate reaction showed that the kinetic mechanism for both isoenzymes involved ternarycomplex formation. Double reciprocal plots of initial rates versus the concentration of either one of the two substrates at different constant concentrations of the other substrate were linear and intersected on AGO' = -45 kJImol. This methane-forming reaction is the terminal step in the energy metabolism of all methanogenic Archaea (Wolfe, 1991 ;Ferry, 1992;Weiss and Thauer, 1993). In addition, it is probably the rate-limiting step in methanogenesis. The specific activity of MCR, determined in cell extracts of methanogenic Archaea, is much lower than that of the other catabolic enzymes of the methane-forming pathway (Schworer and Thauer, 1991). The MCR concentration in methanogenic Archaea is relatively high. It can constitute more than 10% of the cellular protein (Rouvikre et al., 1988).It was discovered recently that Methanobacterium thermoautotrophicum, which grows on H, and C02, contains two isoenzymes of MCR, designated MCR I and MCR I1 (Rospert et al., 1990). The two isoenzymes, which can be separated by anion-exchange chromatography on a Mono-Q column, resemble each other in being composed of three different subunits in an (xzp2j~2 arrangement and in containing 2 mol of the nickel porphinoid F430 as tightly, but not covalently bound, prosthetic group. They differ significantly, however, in the size of the y subunit and in the amino acid sequences of all subunits, as revealed by a comparison of their N-terminal amino acid sequences and of their immunological properties. Southern-blot analyses indicate the presence of two related but clearly separate MCR operons in the genomic DNA (Reeve, 1992). Evidence has been presented that MCR I is the predominant form when growth of the methanogen is limited by the rate of the H, and CO, supply and that MCR I1 predominates when the concentration of H2 and of CO, is not growth-rate limiting (Bonacker et al., 1992). It was not clear why this Archaeon uses different iso-