Carbon/Nitrogen Metabolic Balance: Lessons from Cyanobacteria

• Published in: Trends in plant science 2018-12, Vol.23 (12), p.1116-1130
• Main Authors: Zhang, Cheng-Cai, Zhou, Cong-Zhao, Burnap, Robert L., Peng, Ling
• Format: Article
• Language: English
• Subjects: Bacteriology; BASIC BIOLOGICAL SCIENCES; Biofuels; Carbon; Carbon - metabolism; carbon fixation; Cell Respiration; Cellular biology; Crop production; Cyanobacteria; Cyanobacteria - metabolism; Ketoglutaric acid; Krebs cycle; Life Sciences; Metabolic Networks and Pathways; Metabolism; Microbiology and Parasitology; Nitrogen; Nitrogen - metabolism; Oxygenase; photorespiration; Photosynthesis; Ribulose-1,5-bisphosphate; Ribulose-bisphosphate carboxylase; Signal Transduction; Tricarboxylic acid cycle
• ISSN: 1360-1385; 1878-4372
• DOI: 10.1016/j.tplants.2018.09.008

Carbon and nitrogen are the two most abundant nutrient elements for all living organisms, and their metabolism is tightly coupled. What are the signaling mechanisms that cells use to sense and control the carbon/nitrogen (C/N) metabolic balance following environmental changes? Based on studies in cyanobacteria, it was found that 2-phosphoglycolate derived from the oxygenase activity of Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) and 2-oxoglutarate from the Krebs cycle act as the carbon- and nitrogen-starvation signals, respectively, and their concentration ratio likely reflects the status of the C/N metabolic balance. We will present and discuss the regulatory principles underlying the signaling mechanisms, which are likely to be conserved in other photosynthetic organisms. These concepts may also contribute to developments in the field of biofuel engineering or improvements in crop productivity. Cyanobacteria are simple models among photosynthetic organisms for studying the metabolic regulation of carbon/nitrogen (C/N), the two most abundant nutrient elements for all living organisms. 2-Oxoglutarate (2-OG), an intermediate from the Krebs cycle, serves as a carbon skeleton for nitrogen assimilation and as a signal of nitrogen limitation, whereas 2-phosphoglycolate (2-PG), an intermediate from photorespiration, acts as a signal of inorganic carbon limitation. The levels of 2-PG and 2-OG are inversely correlated, and their ratio reflects the C/N metabolic balance. The transcriptional activator NtcA is a sensor of 2-OG, and the transcriptional repressor NdhR is a sensor of both 2-OG and 2-PG. These regulators together balance C/N metabolic networks by switching on or off the expression of genes involved mainly in the uptake and assimilation of carbon and nitrogen sources. The signaling role of 2-OG and 2-PG in C/N balance is likely conserved in other photosynthetic organisms.