Mutants of Azotobacter vinelandii altered in the regulation of nitrate assimilation

Nitrate assimilation by bacteria

Nitrate is a significant nitrogen source for plants and microorganisms. Recent molecular genetic analyses of representative bacterial species have revealed structural and regulatory genes responsible for the nitrate-assimilation phenotype. Together with results from physiological and biochemical studies, this information has unveiled fundamental aspects of bacterial nitrate assimilation and provides the foundation for further investigations. Well-studied genera are: the cyanobacteria, including the unicellular Synechococcus and the filamentous Anabaena; the gamma-proteobacteria Klebsiella and Azotobacter; and a Gram-positive bacterium, Bacillus. Nitrate uptake in most of these groups seems to involve a periplasmic binding protein-dependent system that presumably is energized by ATP hydrolysis (ATP-binding cassette transporters). However, Bacillus may, like fungi and plants, utilize electrogenic uptake through a representative of the major facilitator superfamily of transport proteins. Nitrate reductase contains both molybdenum cofactor and an iron-sulfur cluster. Electron donors for the enzymes from cyanobacteria and Azotobacter are ferredoxin and flavodoxin, respectively, whereas the Klebsiella and Bacillus enzymes apparently accept electrons from a specific NAD(P)H-reducing subunit. These subunits share sequence similarity with the reductase components of bacterial aromatic ring-hydroxylating dehydrogenases such as toluene dioxygenase. Nitrite reductase contains sirohaem and an iron-sulfur cluster. The enzymes from cyanobacteria and plants use ferredoxin as the electron donor, whereas the larger enzymes from other bacteria and fungi contain FAD and NAD(P)H binding sites. Nevertheless, the two forms of nitrite reductase share recognizable sequence and structural similarity. Synthesis of nitrate assimilation enzymes and uptake systems is controlled by nitrogen limitation in all bacteria examined, but the relevant regulatory proteins exhibit considerable structural and mechanistic diversity in different bacterial groups. A second level of control, pathway-specific induction by nitrate and nitrite in Klebsiella, involves transcription antitermination. Several issues await further experimentation, including the mechanism and energetics of nitrate uptake, the pathway(s) for nitrite uptake, the nature of electron flow during nitrate reduction, and the action of transcriptional regulatory circuits. Fundamental knowledge of nitrate assimilation physiology should also enhance the study of nitrate metabolism in soil, water and other natural environments, a challenging topic of considerable interest and importance.

Positive regulation by nitrite of the nitrate assimilation operon in the cyanobacteria Synechococcus sp. strain PCC 7942 and Plectonema boryanum

In the absence of fixation of ammonium to glutamine, nitrate and nitrite activated transcription of the nitrate assimilation (nirA-nrtABCD-narB) operon of Synechococcus sp. strain PCC 7942. In a nitrate reductase-deficient mutant, only nitrite activated transcription, indicating that nitrite is the actual activator of the operon. Nitrate and nitrite were also found to activate the transcription of a nitrate assimilation operon in the filamentous nonheterocystous nitrogen-fixing cyanobacterium Plectonema boryanum.

A sensor protein involved in induction of nitrate assimilation in Azotobacter chroococcum

Nitrogen-fixing Azotobacter chroococcum cells, but not ammonium- or nitrate-grown cells, exhibited two polypeptide components of 22 and 35 kDa, respectively, that we termed P22 and P35. Bidimensional polyacrylamide gel electrophoresis analysis of preparations from N2-fixing cells that had been transferred to nitrate medium and then incubated for 2 h revealed that P22 had shifted to a more acidic part of the gel while P35 did not change its electrophoretic pattern. Using [32P]orthophosphoric acid it could be demonstrated that the shift in mobility of P22 was due to the phosphorylation of the polypeptide dependent on nitrate (nitrite). The A. chroococcum TR1 strain, which is unable to use nitrate as a nitrogen source and displays activities of nitrogenase, nitrate reductase and nitrite reductase, exhibited both polypeptides. In contrast, P22 and P35 were absent from A. chroococcum MCD1, a mutant strain that cannot assimilate nitrate and lacks the nitrate-reducing enzymatic system. The results suggest that P22 could act as a sensor protein for nitrate in A. chroococcum.

Identification of an operon involved in the assimilatory nitrate-reducing system of Azotobacter vinelandii

A number of mutants lacking nitrate reductase (Nas-) or nitrite reductase (Nis-) activities have been isolated and characterized. An operon including two new genes (nasA and nasB) has been defined and cloned from an Azotobacter vinelandii gene bank. nasA encodes for nitrite reductase apoenzyme, whereas nasB is specific for nitrate reductase activity. Nitrate reductase exerts a regulatory effect on nasAB.

The product of the nitrogen fixation regulatory gene nfrX of Azotobacter vinelandii is functionally and structurally homologous to the uridylyltransferase encoded by glnD in enteric bacteria

We sequenced the nitrogen fixation regulatory gene nfrX from Azotobacter vinelandii, mutations in which cause a Nif- phenotype, and found that it encodes a 105-kDa protein (NfrX), the N terminus of which is highly homologous to that of the uridylyltransferase-uridylyl-removing enzyme encoded by glnD in Escherichia coli. In vivo complementation experiments demonstrate that the glnD and nfrX products are functionally interchangeable. A vinelandii nfrX thus appears to encode a uridylyltransferase-uridylyl-removing enzyme, and in this paper we report the first sequence of such a protein. The Nif- phenotype of nfrX mutants can be suppressed by a second mutation in a recently identified nifL-like gene immediately upstream of nifA in A. vinelandii. NifL mediates nif regulation in response to the N status in A. vinelandii, presumably by inhibiting NifA activator function as occurs in Klebsiella pneumoniae; thus, one role of NfrX is to modify, either directly or indirectly, the activity of the nifL product.

Characterization of the pyoverdines of Azotobacter vinelandii ATCC 12837 with regard to heterogeneity

Azotobacter vinelandii strain ATCC 12837 produces peptide siderophores of the general class known as pyoverdines. In the past, it was assumed that a single well-defined pyoverdine was produced by each parent microorganism. However, there are a number of reports of incompletely characterized pyoverdines that demonstrate heterogeneity in pyoverdine preparations obtained from a single organism, but the nature of this phenomena has not been explained. This study shows that A. vinelandii does indeed produce more than one pyoverdine and that these compounds differ in their peptide components. The metabolism of these siderophores suggests that only one of them is a true siderophore while the others are metabolic byproducts. It was demonstrated that this phenomenon is likely due to intrinsic limitations of the synthetase complex involved in the biosynthesis of these compounds. Characterization of two of the major pyoverdines produced demonstrated that they are novel compounds, although they belonged to the Azotobacter-type family of pyoverdines.