The nucleotide sequence has been determined for a twelve-gene operon of designated the operon (). The operon is located at 55.8-56.0 min and encodes a putative nine-subunit hydrogenase complex (hydrogenase four or Hyf), a potential formate- and σdependent transcriptional activator, HyfR (related to FhlA), and a possible formate transporter, FocB (related to FocA). Five of the nine Hyf-complex subunits are related to subunits of both the hydrogenase-3 complex (Hyc) and the proton-translocating NADH:quinone oxidoreductases (complex I and Nuo), whereas two Hyf subunits are related solely to NADH:quinone oxidoreductase subunits. The Hyf components include a predicted 523 residue [Ni-Fe] hydrogenase (large subunit) with an N-terminus (residues 1-170) homologous to the 30 kDa or NuoC subunit of complex I. It is proposed that Hyf, in conjunction with formate dehydrogenase H (Fdh-H), forms a hitherto unrecognized respiration-linked proton-translocating formate hydrogenlyase (FHL-2). It is likely that HyfR acts as a formate-dependent regulator of the operon and that FocB provides the Hyf complex with external formate as substrate.

Keyword(s): complex I , FhlA , FocA , formate and hydrogenase

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