Review Photosynthetic nitrate assimilation in cyanobacteria w Enrique Flores 1, *, Jose´ E. Frı´as 1 , Luis M. Rubio 2 & Antonia Herrero 1 1 Instituto de Bioquı´mica Vegetal y Fotosı´ntesis, C.S.I.C.-Universidad de Sevilla, Avda. Ame´rico Vespucio 49, 41092 Sevilla, Spain; 2 Department of Plant and Microbial Biology, 111 Koshland Hall, University of Cali- fornia, Berkeley, CA 94720, USA; *Author for correspondence (e-mail: eflores@ibvf.csic.es; fax: +34- 954460065) Received 26 August 2004; accepted in revised form 2 November 2004 Key words: CnaT, ferredoxin, nitrate permease, nitrate reductase, nitrite reductase, NtcA, NtcB, P II protein, 2-oxoglutarate 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 reg- ulatory mechanisms are likely to be discovered. Introduction The assimilatory reduction of nitrate to ammo- nium is a key step of the nitrogen cycle in the biosphere. Nitrate reduction to ammonium is carried out by many bacteria, fungi, algae and plants, and takes place through two sequential reactions involving 2-electron and 6-electron reductions that are catalyzed, respectively, by nitrate reductase and nitrite reductase. The ammonium resulting from nitrate reduction is incorporated into carbon skeletons via glutamate dehydrogenase or the glutamine synthetase/gluta- mate synthase cycle, the latter being the predomi- nant ammonium assimilation pathway in phototrophic organisms. Because nitrate is fre- quently found in the environment at relatively low concentrations (e.g., in the lM range), specific nitrate uptake systems are required to concentrate this nutrient inside the cells before nitrate reduc- tion can take place. In organisms performing oxygenic photosynthesis, i.e., the cyanobacteria, algae and plants, nitrate reduction can be func- w This article is dedicated to Professor Manuel Losada on the occasion of his 75th anniversary. Photosynthesis Research (2005) 83: 117–133 Ó Springer 2005