HYDROGEN REGULATION OF GROWTH, GROWTH YIELDS, AND METHANE GENE-TRANSCRIPTION IN METHANOBACTERIUM-THERMOAUTOTROPHICUM DELTA-H

Changes in growth rate, methanogenesis, growth yield (Y-CH4), and meth ane gene transcription have been correlated with changes in the supply of H-2 to Methanobacterium thermoautotrophicum Delta H cells growing on H-2 plus CO2 in fed-batch cultures. Under conditions of excess H-2, biomass and methanogenesis increased exponentially and in parallel, r esulting in cultures with a constant Y-CH4 and transcription of the mt h and mrt genes that encode the H-2-dependent N-5,N-10-methenyltetrahy dromethanopterin (methenyl-H(4)MPT) reductase (MTH) and methyl coenzym e M reductase II (MRII), respectively. Reducing the H-2 supply, by dec reasing the percentage of H-2 in the input gas mixture or by reducing the mixing speed of the fermenter impeller, decreased the growth rate and resulted in lower and constant rates of methanogenesis, Under such H-2-limited growth conditions, cultures grew with a continuously incr easing Y-CH4 and the Mtd and mcr genes that encode the reduced coenzym e F-420-dependent N-5,N-10-methenyl-H(4)MPT reductase (MTD) and methyl coenzyme M reductase I (MRT), respectively, were transcribed. Changes in the kinetics of growth, methanogenesis, and methane gene transcrip tion directed by reducing the H-2 supply could be reversed by restorin g a high H-2 supply, Methane production continued, but at a low and co nstant rate, and only mcr transcripts could be detected when the H-2 s upply was reduced to a level insufficient for growth, ftsA transcripts , which encode coenzyme F-390 synthetase, were most abundant in cells growing with high H-2 availability, consistent with coenzyme F-390 syn thesis signaling a high exogenous supply of reductant.