A global two component signal transduction system that integrates the control of photosynthesis, carbon dioxide assimilation, and nitrogen fixation

Photosynthesis, biological nitrogen fixation, and carbon dioxide assimilation are three fundamental biological processes catalyzed by photosynthetic bacteria. In the present study, it is shown that mutant strains of the nonsulfur purple photosynthetic bacteria Rhodospirillum rubrum and Rhodobacter sphaeroides, containing a blockage in the primary CO2 assimilatory pathway, derepress the synthesis of components of the nitrogen fixation enzyme complex and abrogate normal control mechanisms. The absence of the Calvin–Benson–Bassham (CBB) reductive pentose phosphate CO2 fixation pathway removes an important route for the dissipation of excess reducing power. Thus, the mutant strains develop alternative means to remove these reducing equivalents, resulting in the synthesis of large amounts of nitrogenase even in the presence of ammonia. This response is under the control of a global two-component signal transduction system previously found to regulate photosystem biosynthesis and the transcription of genes required for CO2 fixation through the CBB pathway and alternative routes. In addition, this two-component system directly controls the ability of these bacteria to grow under nitrogen-fixing conditions. These results indicate that there is a molecular link between the CBB and nitrogen fixation process, allowing the cell to overcome powerful control mechanisms to remove excess reducing power generated by photosynthesis and carbon metabolism. Furthermore, these results suggest that the two-component system integrates the expression of genes required for the three processes of photosynthesis, nitrogen fixation, and carbon dioxide fixation.

Involvement of Ureides in Nitrogen Fixation Inhibition in Soybean1

The sensitivity of N2 fixation to drought stress in soybean (Glycine max Merr.) has been shown to be associated with high ureide accumulation in the shoots, which has led to the hypothesis that N2 fixation during drought is decreased by a feedback mechanism. The ureide feedback hypothesis was tested directly by measuring the effect of 10 mm ureide applied by stem infusion or in the nutrient solution of hydroponically grown plants on acetylene reduction activity (ARA). An almost complete inhibition of ARA was observed within 4 to 7 d after treatment, accompanied by an increase in ureide concentration in the shoot but not in the nodules. The inhibition of ARA resulting from ureide treatments was dependent on the concentration of applied ureide. Urea also inhibited ARA but asparagine resulted in the greatest inhibition of nodule activity. Because ureides did not accumulate in the nodule upon ureide treatment, it was concluded that they were not directly inhibitory to the nodules but that their influence mediated through a derivative compound, with asparagine being a potential candidate. Ureide treatment resulted in a continual decrease in nodule permeability to O2 simultaneous with the inhibition of nitrogenase activity during a 5-d treatment period, although it was not clear whether the latter phenomenon was a consequence or a cause of the decrease in the nodule permeability to O2.

Azotobacter vinelandii NIFL is a flavoprotein that modulates transcriptional activation of nitrogen-fixation genes via a redox-sensitive switch.

The NIFL regulatory protein controls transcriptional activation of nitrogen fixation (nif) genes in Azotobacter vinelandii by direct interaction with the enhancer binding protein NIFA. Modulation of NIFA activity by NIFL, in vivo occurs in response to external oxygen concentration or the level of fixed nitrogen. Spectral features of purified NIFL and chromatographic analysis indicate that it is a flavoprotein with FAD as the prosthetic group, which undergoes reduction in the presence of sodium dithionite. Under anaerobic conditions, the oxidized form of NIFL inhibits transcriptional activation by NIFA in vitro, and this inhibition is reversed when NIFL is in the reduced form. Hence NIFL is a redox-sensitive regulatory protein and may represent a type of flavoprotein in which electron transfer is not coupled to an obvious catalytic activity. In addition to its ability to act as a redox sensor, the activity of NIFL is also responsive to adenosine nucleotides, particularly ADP. This response overrides the influence of redox status on NIFL and is also observed with refolded NIFL apoprotein, which lacks the flavin moiety. These observations suggest that both energy and redox status are important determinants of nif gene regulation in vivo.

Chloroplast gene sequence data suggest a single origin of the predisposition for symbiotic nitrogen fixation in angiosperms.

Of the approximately 380 families of angiosperms, representatives of only 10 are known to form symbiotic associations with nitrogen-fixing bacteria in root nodules. The morphologically based classification schemes proposed by taxonomists suggest that many of these 10 families of plants are only distantly related, engendering the hypothesis that the capacity to fix nitrogen evolved independently several, if not many, times. This has in turn influenced attitudes toward the likelihood of transferring genes responsible for symbiotic nitrogen fixation to crop species lacking this ability. Phylogenetic analysis of DNA sequences for the chloroplast gene rbcL indicates, however, that representatives of all 10 families with nitrogen-fixing symbioses occur together, with several families lacking this association, in a single clade. This study therefore indicates that only one lineage of closely related taxa achieved the underlying genetic architecture necessary for symbiotic nitrogen fixation in root nodules.

Report of the Public Meeting on Genetic Engineering for Nitrogen Fixation, held at the National Academy of Sciences, Washington, D.C., October 5-6, 1977 ; edited by Alexander Hollaender.

Mode of access: Internet.

Physical and chemical data of nitrogen fixation.

Mode of access: Internet.

Nitrogen fixation in alfalfa : responses to bidirectional selection for associated characteristics /

"Issued December 1981"--2d prelim. page.

Effects of phosphorus on the growth and nitrogen fixation rates of Lyngbya majuscula: implications for management in Moreton Bay, Queensland

Significant acetylene reduction and therefore N-2 fixation was observed for Lyngbya majuscula only during dark periods, which suggests that oxygenic photosynthesis and N-2 fixation are incompatible processes for this species. Results from a series of batch and continuous-flow-culture reactor studies showed that the specific growth rate and N-2 fixation rate of L, majuscula increased with phosphate (P-PO4) concentration up to a maximum value and thereafter remained constant. The P-PO4 concentrations corresponding to the maximum N-2 fixation and maximum growth rates were -0.27 and -0.18 muM respectively and these values are denoted as the saturation values for N-2 fixation and growth respectively. Regular monitoring studies in Moreton Bay, Queensland, show that concentrations Of P-PO4 generally exceed these saturation values over a large portion of the Bay and therefore, the growth of the bloom-forming L, majuscula is potentially maximised throughout much of the Bay by the elevated P-PO4 concentrations. Results from other studies suggest that the elevated P-PO4 concentrations in the Bay can be largely attributed to discharges from waste-water treatment plants (WWTPs), and thus it is proposed that the control of the growth of L. majuscula in Moreton Bay will require a significant reduction in the P load from the WWTP discharges. If the current strategy of N load reduction for these discharges is maintained in the absence of substantial P load reduction, it is hypothesised that the growth of L, majuscula and other diazotrophs in Moreton Bay will increase in the future.