Photosynthetic nitrate assimilation in cyanobacteria

Abstract

Nitrate uptake and reduction to nitrite and ammonium are driven in cyanobacteria by photosynthetically generated assimilatory power, i.e., ATP and reduced ferredoxin. High-affinity nitrate and nitrite uptake takes place in different cyanobacteria through either an ABC-type transporter or a permease from the major facilitator superfamily (MFS). Nitrate reductase and nitrite reductase are ferredoxin-dependent metalloenzymes that carry as prosthetic groups a [4Fe-4S] center and Mo-bis-molybdopterin guanine dinucleotide (nitrate reductase) and [4Fe-4S] and siroheme centers (nitrite reductase). Nitrate assimilation genes are commonly found forming an operon with the structure: nir (nitrite reductase)-permease gene(s)-narB (nitrate reductase). When the cells perceive a high C to N ratio, this operon is transcribed from a complex promoter that includes binding sites for NtcA, a global nitrogen-control regulator that belongs to the CAP family of bacterial transcription factors, and NtcB, a pathway-specific regulator that belongs to the LysR family of bacterial transcription factors. Transcription is also affected by other factors such as CnaT, a putative glycosyl transferase, and the signal transduction protein P(II). The latter is also a key factor for regulation of the activity of the ABC-type nitrate/nitrite transporter, which is inhibited when the cells are incubated in the presence of ammonium or in the absence of CO(2). Notwithstanding significant advance in understanding the regulation of nitrate assimilation in cyanobacteria, further post-transcriptional regulatory mechanisms are likely to be discovered.

The relationship between redox enzyme activity and electrochemical potential—cellular and mechanistic implications from protein film electrochemistry

“…However, they are ideally poised to be engaged by the low potential ferredoxin redox partner that receives electrons from photosystem I. 43 Thus, it may be a general phenomenon that the activity-potential relationship of a redox enzyme is tuned to complement potentials imposed by the redox poise of the electron transfer partner and so facilitate cellular function.…”

Hfq Is Required for Optimal Nitrate Assimilation in the Cyanobacterium Anabaena sp. Strain PCC 7120

“…Nitrate and nitrite reductase enzymes require complex metalcontaining prosthetic groups for their correct function (14). Nitrate reductase contains a molybdenum cofactor (Mo cofactor), in the form of Mo-bis-molybdopterin guanine dinucleotide (25,41).…”

“…The biosynthesis of the Mo cofactor requires the product of at least seven genes (moaA, moaC, moaD, moaE, moeA, moeB, and mobA) (29). Nitrite reductase contains a siroheme prosthetic group (14), and the cysG gene encoding a general siroheme-synthesizing enzyme is clustered with the nir gene in some cyanobacteria (e.g., Synechococcus sp. strains WH 8102 and WH 8103 [2,14]).…”

“…Also, genes all0601 and all0605, encoding CnaT and NirB, respectively, are involved in this regulation (18,21). NtcA, NtcB, and CnaT are positive regulators of the operon, whereas NirB and NirA are negative regulators of the operon and are involved in the induction of the operon by nitrate observed in N 2 -fixing cyanobacteria (14,18).…”

Cytomorphological and nitrogen metabolic enzyme analysis of psychrophilic and mesophilic Nostoc sp.: a comparative outlook

“…4 Growth relationship of psychrophilic and mesophilic Nostoc strains at different temperatures a dry weight, b chlorophyll, c carotenoids, and d protein reactions involving 2-electron and 6-electron reductions and the resulting ammonium ion is incorporated into various intracellular components such as amino acids, protein, RNA and chlorophyll via glutamine synthetase/glutamate synthase (GS-GOGAT) pathway (Flores et al 2005). In the present study, NR activity reliably got diminished at a lower temperature when compared to the higher temperature.…”