We measured F420-dependent N5, N-10-methylenetetrahydro-methanopterin dehydrogenase, N5, N-10-methenyltetrahydro-methanopterin cyclohydrolase, and F420-reducing hydrogenase levels in Methanosarcina barkeri grown on various substrates. Variation in dehydrogenase levels during growth on a specific substrate was usually < 3-fold, and much less for cyclohydrolase. H2-CO2-, methanol-, and H2-CO2+ methanol-grown cells had roughly equivalent levels of dehydrogenase and cyclohydrolase. In acetate-grown cells cyclohydrolase level was lowered 2 to 3-fold and dehydrogenase 10 to 80-fold; this was not due to repression by acetate, since, if cultures growing on acetate were supplemented with methanol or H-2-CO2, dehydrogenase levels increased 14 to 19-fold, and cyclohydrolase levels by 3 to 4-fold. Compared to H-2-CO2- or methanol-grown cells, acetate-or H-2-CO2+methanol-grown cells had lower levels of and less growth phase-dependent variation in hydrogenase activity. Our data are consistent with the following hypotheses: 1. M. barkeri oxidizes methanol via a portion of the CO2-reduction pathway operated in the reverse direction. 2. When steps from CO2 to CH3-S-CoM in the CO2-reduction pathway (in either direction) are not used for methanogenesis, hydrogenase activity is lowered.
