Effect of glucose on cellular response of the cyanobacterium Synechocystis sp. PCC 6803 to nitrogen deficiency

In cyanobacteria, nitrogen deficiency induces the degradation of phycobilisomes, which are then recycled as a nitrogen source. However, this process is suppressed when excess carbon sources are available. To clarify how an imbalance between carbon and nitrogen affects cellular physiology, we examined the effects of glucose on photosynthesis and gene expression under nitrogen-deficient conditions in Synechocystis sp. PCC 6803. Under nitrogen deficiency without glucose, phycobilisome degradation occurred rapidly, allowing >90% cell survival after 72 h. In contrast, with glucose (1 mM), phycobilisome degradation was suppressed, photochemical quenching (an indicator of PSII electron transport) was nearly abolished within 24 h, and cell viability declined sharply. RNA-seq at 4 h showed that under nitrogen deficiency, many photosynthesis-related genes were significantly downregulated, while genes related to carbon catabolic pathways were generally not downregulated so much, regardless of glucose presence. In the presence of glucose, pentose phosphate pathway genes, zwf and gnd , were rather upregulated at 4 h, and by 24 h, a drastic downregulation of a broad set of photosynthesis-related genes was observed, along with the downregulation of genes involved in phycobilisome degradation ( nblA1 , nblA2 , clpC ). These results suggest that under nitrogen deficiency with glucose, carbon catabolism continues for several hours to some extent. This, in turn, suppresses phycobilisome degradation within 24 h, resulting in a buildup of reducing power in the thylakoid membrane. The consequent disruption of electron transport likely causes damage to the photosynthetic machinery and ultimately leads to cell death. • Glucose suppresses phycobilisome degradation under nitrogen-deficient conditions • Electron transport collapses within 24 h under N deficiency with glucose • Pentose phosphate pathway genes are upregulated at 4 h under N deficiency + glucose • Photosynthesis and phycobilisome degradation genes are largely downregulated by 24 h • Accumulated reducing power likely causes photodamage and cell death