Signal transduction histidine kinases (STHK) are key for sensing environmental stresses,
crucial for cell survival, and attain their sensing ability using small molecule binding
domains. The N-terminal domain in an STHK from Nostoc punctiforme is of unknown
function yet is homologous to the central region in soluble guanylyl cyclase (sGC), the main
receptor for nitric oxide (NO). This domain is termed H-NOXA (or H-NOBA) because it is
often associated with the heme-nitric oxide/oxygen binding (H-NOX) domain. A structure …