The NAD salvage pathway enzyme NMNAT-C sustains dark-phase NAD+ homeostasis in cyanobacteria.

Nicotinamide adenine dinucleotide (NAD+) is a crucial cofactor in cyanobacteria, which serve as model organisms for studying photosynthesis. Maintaining NAD+ homeostasis in cyanobacteria is critically important, and it is currently believed that multiple pathways contribute to NAD+ biosynthesis in these organisms. However, the specific contribution of each pathway to NAD+ supplementation under both light and dark conditions, which determines NAD+ homeostasis, has not yet been studied. In this study, we identified NMNAT-C, a cyanobacterial nicotinamide nucleotide adenylyltransferase (NMNAT), as a key player in NAD+ homeostasis, particularly during dark phases. NMNAT-C showed opposite-phase oscillations in expression, aligned with NAD+ fluctuations during light-dark cycles. Genetic and biochemical tests revealed that deleting NMNAT-C in one cyanobacterium (Synechococcus elongatus PCC 7942) accelerated NAD+ depletion during dark periods, increased sensitivity to dark stress, and impacted growth rate. Conversely, induced overexpression of NMNAT-C temporarily raised NAD+ levels but also caused adverse effects over time. Metabolomic analysis indicated that NMNAT-C plays a role in mediating the metabolic crosstalk between the NAD+ salvage pathway and the de novo pathway. Our results identify NMNAT-C as a key regulator of NAD+ dynamics that aligns with daily cycles and suggest that this enzyme plays a crucial role in maintaining NAD+ homeostasis through the NAD+ salvage pathway.