Identification of formate dehydrogenase-specific mRNA species and nucleotide sequence of the fdhC gene of Methanobacterium formicicum

The overlapping fdhA and fdhB genes of Methanobacterium formicicum, which encode the α and β subunits, respectively, of formate dehydrogenase, were cotranscribed as part of an approximately 4.5-kb transcript. An additional gene (fdhC) upstream of fdhA was cotranscribed with fdhA and fdhB. The deduced amino acid sequence suggested that fdhC has the potential to encode a hydrophobic polypeptide with a calculated molecular weight of 29,417. A hydropathy plot of the hypothetical polypeptide indicated several potential membrane-spanning regions. The putative fdhC gene product had 28% identity with the deduced amino acid sequence of the nirC gene from Salmonella typhimurium. Northern (RNA) blot analyses and primer extension assays located a transcription start site 268 bp upstream of the initiation codon of fdhC. A sequence identical to the consensus promoter sequence for methanogenic organisms was situated between -35 and -25 bp from the proposed transcription start site. In addition to the 4.5-kb transcript, Northern blot analyses detected a 1.1-kb transcript with an fdhC-specific probe and a 3.4-kb transcript with either an fdhA- or fdhB-specific probe. The levels of all three transcripts were significantly greater in cells grown in media supplemented with molybdate.