GROWTH-DEPENDENT AND SUBSTRATE-DEPENDENT TRANSCRIPTION OF THE FORMATEDEHYDROGENASE (FDHCAB) OPERON IN METHANOBACTERIUM-THERMOFORMICICUM Z-245

The formate dehydrogenase-encoding fdhCAB operon and flanking genes ha ve been cloned and sequenced from Methanobacterium thermoformicicum Z- 245. fdh transcription was shown to be initiated 21 bp upstream from f dhC, although most fdh transcripts terminated or were processed betwee n fdhC and fdhA. The resulting fdhC, fdhAB, and fdhCAB transcripts wer e present at all growth stages in cells growing on formate but were ba rely detectable during early exponential growth on H-2 plus CO2. The l evels of the fdh transcripts did, however, increase dramatically in ce lls growing on H-2 plus CO2, coincident with the decrease in the growt h rate and the onset of constant methanogenesis that occurred when cul ture densities reached an optical density at 600 nm of similar to 0.5. The mth transcript that encodes the H-2-dependent methenyl-H-4 MPT re ductase (MTH) and the frh and mvh transcripts that encode the coenzyme F-420-reducing (FRH) and nonreducing (MVH) hydrogenases, respectively , were also present in cells growing on formate, consistent with the s ynthesis of three hydrogenases, MTH, FRH, and MVH, in the absence of e xogenously supplied H-2. Reducing the H-2 supply to M. thermoformicicu m cells growing on H-2 plus CO2 reduced the growth rate and CH4 produc tion but increased frh and fdh transcription and also increased transc ription of the mtd, mer, and mcr genes that encode enzymes that cataly ze steps 4, 5, and 7, respectively, in the pathway of CO2 reduction to CH4. Reducing the H-2 supply to a level insufficient for growth resul ted in the disappearance of all methane gene transcripts except the mc r transcript, which increased, Regions flanking the fdhCAB operon in M . thermoformicicum Z-245 were used as probes to clone the homologous r egion from the Methanobacterium thermoautotrophicum Delta H genome. Se quencing revealed the presence of very similar genes except that the g enome of M. thermoautotrophicum, a methanogen incapable of growth on f ormate, lacked the fdhCAB operon.