Photosynthetic nitrate assimilation in cyanobacteria.
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.
Flores
Enrique
E
Instituto de Bioquímica Vegetal y Fotosíntesis, C.S.I.C.-Universidad de Sevilla, Avda. Américo Vespucio 49, Seville 41092, Spain. eflores@ibvf.csic.es
Frías
José E
JE
Rubio
Luis M
LM
Herrero
Antonia
A
D001426:Bacterial Proteins / Q000096:biosynthesis; D001693:Biological Transport, Active; D000458:Cyanobacteria / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial; D009566:Nitrates / Q000378:metabolism*; D009573:Nitrites / Q000378:metabolism; D010788:Photosynthesis / Q000502:physiology*; D011401:Promoter Regions, Genetic; D000644:Quaternary Ammonium Compounds / Q000378:metabolism
D001426:Bacterial Proteins; D009566:Nitrates; D009573:Nitrites; D000644:Quaternary Ammonium Compounds