It is of fundamental importance to study and to understand, from a molecular to cellular level, how bacteria respond to their environment. Dissolved gasses (e.g., nitric oxide, NO) are increasingly being recognized as a critical stimuli to organisms. In eukaryotes, the heme sensor protein soluble guanylate cyclase (sGC) is responsible for recognizing and responding to NO. This important enzyme is a member of a broader family of proteins called the Heme-Nitric oxide/Oxygen binding family (H-NOX), which also includes bacterial heme proteins. We have characterized the ligand binding properties of the H-NOX domain from the psychrophile, Colwellia psychrerythraea (Cp) using UV/VIS spectroscopy and found that Cp-H-NOX selectively binds NO with an affinity similar to sGC. In the Cp genome, Cp-H-NOX is located directly upstream from a soluble histidine kinase and a response regulator, prototypical of proteins involved in two component signaling (also known as His-Asp phosphorelay signaling). We are investigating the possibility that Cp responds to NO binding to its H-NOX domain through a His-Asp phosphorelay mechanism. Kinetic and spectroscopic data will be presented to support this hypothesis.