A 12-CISTRON ESCHERICHIA-COLI OPERON (HYF) ENCODING A PUTATIVE PROTON-TRANSLOCATING FORMATE HYDROGENLYASE SYSTEM

The nucleotide sequence has been determined for a twelve-gene operon o f Escherichia coli designated the hyf operon (hyfABCDEFGHIR-focB). The hyf operon is located at 55.8-56.0 min and encodes a putative nine-su bunit hydrogenase complex (hydrogenase four or Hyf), a potential forma te-and sigma(54)-dependent transcriptional activator, HyfR (related to FhlA), and a possible formate transporter, FocB (related to FocA). Fi ve of the nine Hyf-complex subunits are related to subunits of both th e E. coli hydrogenase-3 complex (Hyc) and the proton-translocating NAD H:quinone oxidoreductases (complex I and Nuo), whereas two Hyf subunit s 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 re spiration-linked proton-translocating formate hydrogenlyase (FHL-2). I t is likely that HyfR acts as a formate-dependent regulator of the hyf operon and that FocB provides the Hyf complex with external formate a s substrate.