The genes encoding the two structural subunits of Escherichia coli hydrogenase 2 (HYD2) have been cloned and sequenced. They occur in an operon (hyb) which contains seven open reading frames. An hyb deletion mutant (strain AP3) failed to grow on dihydrogen-fumarate medium and also produced very low levels of HIYD1. All seven open reading frames are required for restoration of wild-type levels of active HYD2 in AP3.The hyb operon was mapped at 65 min on the E. coli chromosome.Under anaerobic growth conditions, Escherichia coli produces three different nickel-containing hydrogenases (3, 39). Hydrogenase 3 (HYD3) is part of the formate hydrogenlyase (FHL) complex and is responsible for formate-dependent dihydrogen (H2) evolution. The operon encoding HYD3 and other accessory electron transport components of the FHL complex, hyc, has been identified and is located at 58 min on the E. coli chromosome (7). The highly oxygen-labile nature of HYD3 has precluded detailed biochemical characterization. HYD2 is involved in H2 uptake and can be differentially induced to high levels when cells are grown in medium containing H2 as an electron donor and fumarate as an electron acceptor (3,23,25,39). An active component of HYD2 has been purified and shown to be a heterodimeric enzyme with a 58-kDa large subunit and a 30-kDa small subunit (4). Although mutants defective in H2 uptake have been described (23,25), detailed analysis of the operon encoding HYD2 has not been carried out. HYDl has also been purified and shown to consist of a large (60 kDa) subunit and a small (30 kDa) subunit (16,40). An active form of HYD1 containing only the large subunit has also been purified and characterized (1, 16). The operon encoding the two structural subunits of HYD1 (hya) contains a total of six genes and has been mapped at 22 min on the E. coli chromosome (30,31). The function of HYD1 is not understood, but it is believed to have a role in hydrogen cycling during fermentative growth. In addition to the operons coding for the structural components of the three hydrogenases, a fourth operon, hyp, located at 58 min, is essential for activity of all three hydrogenases (20,26,38). At least one of the genes in this operon (hypB) is involved in nickel metabolism, most probably via nickel insertion into apoenzyme (27).In this paper, we present the DNA sequence of the operon encoding HYD2 (hyb), which contains seven open reading frames (ORFs). Cassette mutagenesis of the hyb operon on the chromosome resulted in a total loss of HYD2 expression and activity, as well as in significant reduction in HYD1 activity. MATERIALS AND METHODSBacterial strains. All bacterial strains used were E. coli K-12 derivatives and are listed in Table 1 [pH 7.0]), resuspended in the same buffer to an optical density of 0.5 at 600 nm, and used for whole-cell enzyme assays. Cell extracts were prepared by sonicating cell suspensions on ice with a model W385 sonicator (Heat Systems) for 20 5-s bursts. Triton X-100 was added to a final concentration of 2% (vol/vol), when req...

DNA encompassing the structural genes of an Escherichia coli [NiFe] hydrogenase has been cloned and sequenced. The genes were identified as those encoding the large and small subunits of hydrogenase isozyme 1 based on NH2-terminal sequences of purified subunits (kindly provided by K. Francis and K. T. Shanmugam). The structural genes formed part of a putative operon that contained four additional open reading frames. We have designated the operon hya and the six open reading frames hyaA through F. hyaA and hyaB encode the small and large structural subunits, respectively. The nucleotide-derived amino acid sequence of hyaC has a calculated molecular mass of 27.6 kilodaltons, contains 20% aromatic residues, and has four potential membrane-spanning regions. Open reading frames hyaD through F could encode polypeptides of 21.5, 14.9, and 31.5 kilodaltons, respectively. These putative peptides have no homology to other reported protein sequences, and their functions are unknown.The anaerobic hydrogen metabolism of Escherichia coli and other enterobacteria is intricately regulated with regard to both hydrogen production during fermentation and hydrogen oxidation during anaerobic respiration (13,14). This complex regulatory system responds to specific substrates as well as to global regulatory signals and results in the biosynthesis of discrete hydrogenases specific to a given metabolic pathway. Three E. coli hydrogenases have been described which are synthesized in response to different physiological conditions. Hydrogenases 1 and 2 have been biochemically characterized and are immunologically distinct membranebound nickel-containing proteins (2, 3, 31). The existence of hydrogenase 3 was originally inferred from the fact that hydrogenases 1 and 2 immunoprecipitated from cell lysates did not account for total hydrogenase activity (30). Hydrogenase 3 activity is very labile and has been only partially characterized (34). Hydrogenases 1 and 3 are induced to higher levels by growth on glucose and formate. Hydrogenase 3 has also been shown to have a role in the formate hydrogenlyase pathway and accounts for about 60 to 70% of the total hydrogenase activity in the cell. The physiological role of hydrogenase 1 has not been defined. Hydrogenase 2 has been implicated as a respiratory uptake hydrogenase coupled to fumarate reduction and is induced to a higher level by growth in the presence of glycerol and fumarate (30).The presence of three different enzymes catalyzing the same reactions makes the biochemical, physiological, and genetic analyses of their metabolic roles technically difficult. Toward understanding the physiological roles of the different hydrogenases, a large number of E. coli mutants defective in hydrogenase activities have been analyzed. Most of these mutants lack all three hydrogenase activities; based on genetic analysis, the mutations appear to be exclusively at loci affecting regulation and do not encompass the hydrogenase structural genes. These genetic loci have been designated as hydA through F (8, 16...

Batch cultures of Desulfovibrio vulgaris stored at 32°C for 10 months have been found to retain 50% of the hydrogenase activity of a 1-day culture. The hydrogenase found in old cultures needs reducing conditions for its activation. Viable cell counts are negative after 6 months, showing that the hydrogenase activity does not depend on the presence of viable cells. These observations are of importance in the understanding of anaerobic biocorrosion of metals caused by depolarization phenomena.

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