A singular nitric oxide synthase with a globin domain found in Synechococcus PCC 7335 mobilizes N from arginine to nitrate.

The enzyme nitric oxide synthase (NOS) oxidizes L-arginine to NO and citrulline. In this work， we characterise the NOS from the cyanobacteria Synechococcus PCC 7335 (SyNOS). SyNOS possesses a canonical mammalian NOS architecture consisting of oxygenase and reductase domains. In addition， SyNOS possesses an unusual globin domain at the N-terminus. Recombinant SyNOS expressed in bacteria is active， and its activity is suppressed by the NOS inhibitor L-NAME. SyNOS allows E. coli to grow in minimum media containing L-arginine as the sole N source， and has a higher growth rate during N deficiency. SyNOS is expressed in Synechococcus PCC 7335 where NO generation is dependent on L-arginine concentration. The growth of Synechococcus is dramatically inhibited by L-NAME， suggesting that SyNOS is essential for this cyanobacterium. Addition of arginine in Synechococcus increases the phycoerythrin content， an N reservoir. The role of the novel globin domain in SyNOS is discussed as an evolutionary advantage， conferring new functional capabilities for N metabolism.