Analysis of a gene region required for dihydrogen oxidation in Azotobacter vinelandii

Azotobacter vinelandii: the source of 100 years of discoveries and many more to come

Azotobacter vinelandii has been studied for over 100 years since its discovery as an aerobic nitrogen-fixing organism. This species has proved useful for the study of many different biological systems, including enzyme kinetics and the genetic code. It has been especially useful in working out the structures and mechanisms of different nitrogenase enzymes, how they can function in oxic environments and the interactions of nitrogen fixation with other aspects of metabolism. Interest in studying A. vinelandii has waned in recent decades, but this bacterium still possesses great potential for new discoveries in many fields and commercial applications. The species is of interest for research because of its genetic pliability and natural competence. Its features of particular interest to industry are its ability to produce multiple valuable polymers - bioplastic and alginate in particular; its nitrogen-fixing prowess, which could reduce the need for synthetic fertilizer in agriculture and industrial fermentations, via coculture; its production of potentially useful enzymes and metabolic pathways; and even its biofuel production abilities. This review summarizes the history and potential for future research using this versatile microbe.

Aerobic Hydrogen Production via Nitrogenase in Azotobacter vinelandii CA6

The diazotroph Azotobacter vinelandii possesses three distinct nitrogenase isoenzymes, all of which produce molecular hydrogen as a by-product. In batch cultures, A. vinelandii strain CA6, a mutant of strain CA, displays multiple phenotypes distinct from its parent: tolerance to tungstate, impaired growth and molybdate transport, and increased hydrogen evolution. Determining and comparing the genomic sequences of strains CA and CA6 revealed a large deletion in CA6's genome, encompassing genes related to molybdate and iron transport and hydrogen reoxidation. A series of iron uptake analyses and chemostat culture experiments confirmed iron transport impairment and showed that the addition of fixed nitrogen (ammonia) resulted in cessation of hydrogen production. Additional chemostat experiments compared the hydrogen-producing parameters of different strains: in iron-sufficient, tungstate-free conditions, strain CA6's yields were identical to those of a strain lacking only a single hydrogenase gene. However, in the presence of tungstate, CA6 produced several times more hydrogen. A. vinelandii may hold promise for developing a novel strategy for production of hydrogen as an energy compound.

In vivo nickel insertion into the carbon monoxide dehydrogenase of Rhodospirillum rubrum: molecular and physiological characterization of cooCTJ

The products of cooCTJ are involved in normal in vivo Ni insertion into the carbon monoxide dehydrogenase (CODH) of Rhodospirillum rubrum. Located on a 1.5-kb DNA segment immediately downstream of the CODH structural gene (cooS), two of the genes encode proteins that bear motifs reminiscent of other (urease and hydrogenase) Ni-insertion systems: a nucleoside triphosphate-binding motif near the N terminus of CooC and a run of 15 histidine residues regularly spaced over the last 30 amino acids of the C terminus of CooJ. A Gm(r)omega-linker cassette was developed to create both polar and nonpolar (60 bp) insertions in the cooCTJ region, and these, along with several deletions, were introduced into R. rubrum by homologous recombination. Analysis of the exogenous Ni levels required to sustain CO-dependent growth of the R. rubrum mutants demonstrated different phenotypes: whereas the wild-type strain and a mutant bearing a partial cooJ deletion (of the region encoding the histidine-rich segment) grew at 0.5 microM Ni supplementation, strains bearing Gm(r)omega-linker cassettes in cooT and cooJ required approximately 50-fold-higher Ni levels and all cooC insertion strains, bearing polar or nonpolar insertions, grew optimally at 550 microM Ni.