A shared basis for nutrient limitation response in cyanobacteria

Cyanobacteria possess diverse regulatory mechanisms to adapt to nutrient limitation, yet the extent to which these responses are shared across different nutrient stresses remains unclear. Understanding these commonalities can reveal fundamental principles of cellular resource allocation and survival strategies. In this work, we investigated the transcriptional responses of Synechocystis sp. PCC 6803 to nitrogen, sulfur, or phosphate limitation and found a core set of genes consistently regulated across all three conditions. This shared response includes repression of genes related to photosynthesis and respiratory electron transport, as well as genes encoding components of the Calvin-Benson cycle, ribosome function, and cellular metabolism. Amongst the highest affected pathways is chlorophyll biosynthesis. A subset of regulatory genes, mostly kinases, are upregulated under all three limitation conditions. These results were further validated by a study of the composition and the function of the photosynthetic machinery. Chlorophyll accumulation was arrested immediately upon transition to limiting media, photosynthetic activities were reduced, and protein complexes were degraded. Our findings reveal a conserved program in cyanobacteria that modulates cellular metabolism and photosynthesis in response to diverse nutrient limitations. Based on these findings, we suggest that chlorophyll biosynthesis is a key regulated pathway driving structural and physiological responses in photosynthesis.