Diversity of H2/CO2-Utilizing Acetogenic Bacteria from Fecesof Non-Methane-Producing Humans

The purpose of this work was to study H₂/CO₂-utilizing acetogenic population in the colons of non-methane-producing individuals harboring low numbers of methanogenic archaea. Among the 50 H₂-consuming acetogenic strains isolated from four fecal samples and an in vitro semi-continuous culture enrichment, with H₂/CO₂ as sole energy source, 20 were chosen for further studies. All isolates were Gram-positive strict anaerobes. Different morphological types were identified, providing evidence of generic diversity. All acetogenic strains characterized used H₂/CO₂ to form acetate as the sole metabolite, following the stoichiometric equation of reductive acetogenesis. These bacteria were also able to use a variety of organic compounds for growth. The major end product of glucose fermentation was acetate, except for strains of cocci that mainly produced lactate. Yeast extract was not necessary, but was stimulatory for growth and acetogenesis from H₂/CO₂. Received: 28 December 1995 / Accepted: 30 January 1996     

Nitrate reductases inEscherichia coli

Escherichia coli expresses two different membrane-bound respiratory nitrate reductases, nitrate reductase A (NRA) and nitrate reductase Z (NRZ). In this review, we compare the genetic control, biochemical properties and regulation of these two closely related enzyme systems. The two enzymes are encoded by distinct operons located within two different loci on theE. coli chromosome. ThenarGHJI operon, encoding nitrate reductaseA, is located in thechlC locus at 27 minutes, along with several functionally related genes:narK, encoding a nitrate/nitrite antiporter, and thenarXL operon, encoding a nitrate-activated, two component regulatory system. ThenarZYWV operon, encoding nitrate reductase Z, is located in thechlZ locus located at 32.5 minutes, a region which includes anarK homologue,narU, but no apparent homologue to thenarXL operon. The two membrane-bound enzymes have similar structures and biochemical properties and are capable of reducing nitrate using normal physiological substrates. The homology of the amino acid sequences of the peptides encoded by the two operons is extremely high but the intergenic regions share no related sequences. The expression of both thenarGHJI operon and thenarK gene are positively regulated by two transacting factors Fnr and NarL-Phosphate, activated respectively by anaerobiosis and nitrate, while thenarZYWV operon and thenarU gene are constitutively expressed. Nitrate reductase A, which accounts for 98% of the nitrate reductase activity when fully induced, is clearly the major respiratory nitrate reductase inE. coli while the physiological role of the constitutively expressed nitrate reductase Z remains to be defined.Abbreviations NR nitrate reductase On leave from Department of Biochemistry and Molecular Biology, The University of Texas Medical school at Houston, Houston, Texas, 77225, USA