Fig. 3. Domain organization of arthropod NOS. The ORFs of Drosophila (dNOS) and land crab NOS (Gl-NOS) are compared. The oxygenase domain contains heme-binding and tetrahydrobiopterin (H4) domains; the reductase domain contains binding domains for FMN, FAD and NADPH. A calmodulin (CaM) binding domain is located between the oxygenase and reductase domains and is involved in dimerization and regulation of catalytic activity (Regulski and Tully, 1995; Stasiv et al., 2001). dNOS has a Gln-rich sequence near the amino terminus.