Expression of a Nitric Oxide Synthase Modifies Arginine Metabolism and Accelerates the Response to Nitrogen Deficiency in Cyanobacteria

Nitric oxide (NO) is a signal molecule in organisms of all kingdoms of life, acting as a second messenger during stress responses, growth and developmental processes. Nitric oxide synthases (NOS) are enzymes that produce NO by catalyzing the oxidation of L-arginine to citrulline and NO. The Synechococcus PCC 7335 NOS protein (SyNOS), representative of the cyanobacteria phylum, shows remarkable differences from canonical NOS proteins, with an extra globin domain which has NO dioxygenase activity that can remobilize arginine reserves into nitrate and NO. To study the impact of SyNOS activity on cyanobacterial nitrogen (N) metabolism, a recombinant strain expressing SyNOS under the IPTG-inducible promoter Ptrc (Ptrc-SyNOS) was generated in the model system Synechococcus elongatus PCC 7942. Ptrc-SyNOS cultures showed enhanced NO production and decreased arginine content. When shifted to N deficient conditions, the SyNOS expressing culture reaches a higher optical density due to increased cell size and induces an accelerated metabolic response leading to faster: (i) degradation of phycocyanin, (ii) glycogen accumulation, and (iii) phosphorylation of PII compared to the control Ptrc strain. The results support an important regulatory role of SyNOS to adapt metabolism to environmental N conditions.