A Dual-H-NOX Signaling System in Saccharophagus degradans

Nitric oxide (NO) is a critical signaling molecule involved in the regulation of a wide variety of physiological processes across every domain of life. In most aerobic and facultative anaerobic bacteria, heme-nitric oxide/oxygen binding (H-NOX) proteins selectively sense NO and inhibit the activity of a histidine kinase (HK) located on the same operon. This NO-dependent inhibition of the cognate HK alters the phosphorylation of the downstream response regulators. In the marine bacterium Saccharophagus degradans (Sde), in addition to a typical H-NOX (Sde 3804)/HK (Sde 3803) pair, an orphan H-NOX (Sde 3557) with no associated signaling protein has been identified distant from the H-NOX/HK pair in the genome. The characterization reported here elucidates the function of both H-NOX proteins. Sde 3557 exhibits a weaker binding affinity with the kinase, yet both Sde 3804 and Sde 3557 are functional H-NOXs with proper gas binding properties and kinase inhibition activity. Additionally, Sde 3557 has an NO dissociation rate that is significantly slower than that of Sde 3804, which may confer prolonged kinase inhibition in vivo. While it is still unclear whether Sde 3557 has another signaling partner or shares the histidine kinase with Sde 3804, Sde 3557 is the only orphan H-NOX characterized to date. S. degradans is likely using a dual-H-NOX system to fine-tune the downstream response of NO signaling.