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PubMed1969

Azotobacter vinelandii RNA polymerase. VII. Enzyme transitions during unprimed r[I-C] synthesis.

Transitions in the state of RNA polymerase were demonstrated during the unprimed synthesis of the r[I-C] copolymer. No detectable change in the usual dimer-monomer pattern was noted during the lag phase (0-25 min at 37 degrees ) after analysis of the reaction mixture by acrylamide gel electrophoresis. At the end of the lag phase, a major alteration in the electrophoretic pattern occurred, marked by the disappearance of the dimer-monomer bands and the concomitant appearance of a series of monomer-r[I-C] copolymer complexes. As these complexes of r[I-C] copolymer with one or more polymerase monomer were formed, an enzymatically inactive component (gamma protein) of the polymerase was displaced. During the phase of rapid r[I-C] copolymer synthesis, the active form of the A. vinelandii RNA polymerase was the r[I-C] monomer lacking the gamma protein.

Azotobacter↗
PubMed1972

Electron paramagnetic resonance of nitrogenase and nitrogenase components from Clostridium pasteurianum W5 and Azotobacter vinelandii OP.

The electron paramagnetic resonance of nitrogenase components, separately and together with the other reactants in the nitrogenase system (namely, reductant and Mg.ATP), have been examined at low temperatures (<20 degrees K). The MoFe protein, component I or molybdoferredoxin, in the oxidized (but not oxygen-inactivated) state yields signals with g-values of 4.3, 3.7, and 2.01, and when reduced has no observable electron paramagnetic resonance. The Fe protein, component II, or azoferredoxin, yields a signal with g-values of 2.05, 1.94, and 1.89 in the reduced state that is converted by Mg.ATP into an axial signal with g-values near 2.05 and 1.94, and a second split signal near g = 4.3. The Fe protein has no definite electron paramagnetic resonance in the oxidized (not oxygen-denatured) state under these conditions. The Mg.ATP complex of reduced Fe protein reduces the MoFe protein, whereas dithionite alone does not reduce the MoFe protein. Reoxidation of the system by substrate leads to disappearance of the Fe protein signal and the reappearance of the MoFe protein signal. Thus Mg.ATP, which is hydrolyzed during substrate reduction, converts the Fe protein to a reductant capable of transferring electrons to MoFe protein, after which substrate reduction occurs.

Adenosine Triphosphate↗
PubMed1989

Purification and characterization of the nifN and nifE gene products from Azotobacter vinelandii mutant UW45.

The nifN and -E gene products are involved in the synthesis of the iron-molybdenum cofactor of dinitrogenase, the enzyme responsible for the reduction of dinitrogen to ammonia. By using the in vitro iron-molybdenum cofactor biosynthesis assay, we have followed the purification of these gene products 450-fold to greater than 95% purity. An overall recovery of 20% was obtained with the purified protein having a specific activity of 6900 units/mg of protein. The protein (hereafter referred to as NIFNE) was found to contain equimolar amounts of the nifN and -E gene products and have a native molecular mass of 200 +/- 10 kDa, which indicates an alpha 2 beta 2 structure. NIFNE was oxygen labile with a half-life of 1 min in air. A UV-visible spectrum of the dye-oxidized protein showed an absorption maximum at 425 nm that could be bleached by reduction of NIFNE with sodium dithionite, suggesting the presence of an Fe center in NIFNE.

Azotobacter↗
PubMed2002

Accumulation of 99Mo-containing iron-molybdenum cofactor precursors of nitrogenase on NifNE, NifH, and NifX of Azotobacter vinelandii.

The biosynthesis of the iron-molybdenum cofactor (FeMo-co) of nitrogenase was investigated using the purified in vitro FeMo-co synthesis system and 99Mo. The purified system involves the addition of all components that are known to be required for FeMo-co synthesis in their purified forms. Here, we report the accumulation of a 99Mo-containing FeMo-co precursor on NifNE. Apart from NifNE, NifH and NifX also accumulate 99Mo label. We present evidence that suggests NifH may serve as the entry point for molybdenum incorporation into the FeMo-co biosynthetic pathway. We also present evidence suggesting a role for NifX in specifying the organic acid moiety of FeMo-co.

Adenosine Triphosphate↗
PubMed1987

Properties of allosteric nicotinamide mononucleotide glycohydrolase from Azotobacter vinelandii: activation and inhibition.

In order to clarify the regulation mechanism of NMN glycohydrolase, a number of purine and pyrimidine nucleotides were tested as activators for the enzyme. Among naturally occurring nucleotides, pppGpp was shown to be the most potent activator (KA = 0.0087 mM). The effectiveness of these nucleotides estimated from Vmax/KA was in the order: pppGpp greater than ppGpp greater than pppppG greater than ppppG greater than pGpp greater than GTP greater than or equal to 2'-GMP greater than or equal to GppppG greater than dGTP greater than ITP greater than GDP = 2'-O-methylGTP. XTP, UTP, dTTP gave no effect. In contrast, ATP, CTP, and 7-methylGTP were inhibitory. A comparison of the data indicated that activation of the enzyme is specific for the base structure, guanine, and that effectiveness of nucleotides as activators results from the negatively charged phosphate moiety. Consistent with these results, inorganic polyphosphates (tri-, tetra-, and pentaphosphate) and 5-phosphoribose 1-pyrophosphate were shown to function as inhibitors of the enzyme. The inhibition mechanism was shown to be competitive with GTP. However, apparent Ki values for these inhibitors were dependent on the activator concentration. Furthermore, a shift of cooperativity type and progressive increase of positive cooperativity concomitant with increase of PRPP concentration were observed. These results indicate that this enzyme may be allosteric and an important regulatory component of pyridine nucleotide cycle metabolism.

Allosteric Regulation↗