Nitrogen starvation–induced oxidative stress relieves PII-mediated inhibition of acetyl-CoA carboxylase (ACCase) activity and signals enhanced lipid synthesis in Synechococcus PCC 7942

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The present study shows the existence of PII-acetyl-CoA carboxylase interaction in the cyanobacterium Synechococcus sp. PCC 7942 and the possible adverse impact of nitrogen starvation on this interaction. The in silico and in vitro analysis of PII-acetyl-CoA carboxylase interaction revealed that the biotin carboxyl carrier protein subunit of acetyl-CoA carboxylase enzyme actually interacts with the T-loop of PII protein. However, exposure of the cyanobacterium to nitrogen-starved condition showed a higher expression and activity of acetyl-CoA carboxylase at the intracellular level which denoted the impairment of PII-acetyl-CoA carboxylase interaction. A similar stimulatory effect of nitrogen starvation has also been noticed in the PII mutant of Synechococcus PCC 7942. Further, the physiological study reflected that nitrogen starvation–caused reactive oxygen species generation in the wild-type and PII mutant strains and lipid was increased in both strains of Synechococcus sp. PCC 7942. Proteomic analysis showed the upregulation of glycogen synthase, biotin carboxylase, and antioxidative enzymes and the deregulation of proteins involved in photosynthesis, energy metabolism, and protein synthesis. Interestingly, enhanced accumulation of transcripts of few tricarboxylic acid cycle genes was also noticed in the wild type. Although oxidative stress and lipid production were enhanced in both the test strains under nitrogen starvation, the impacts were more prominent in the mutant strain. Our results suggest that the nitrogen starvation–induced oxidative stress possibly relieved the PII-mediated inhibition of acetyl-CoA carboxylase and led to increased lipid synthesis in Synechococcus PCC 7942.