Nitric oxide synthase (NOS) is amongst a family of evolutionarily conserved enzymes, involved in a multi-turnover process that results in NO as a product. The significant role of NO in various pathological and physiological processes has created an interest in this enzyme from several perspectives. This study describes for the first time, cloning and expression of a NOS-like protein, baNOS, from Bacillus anthracis, a pathogenic bacterium responsible for causing anthrax. baNOS was expressed in Escherichia coli as a soluble and catalytically active enzyme. Homology models generated for baNOS indicated that the key structural features that are involved in the substrate and active site interaction have been highly conserved. Further, the behavior of baNOS in terms of heme-substrate interactions and heme-transitions was studied in detail. The optical perturbation spectra of the heme domain demonstrated that the ligands perturb the heme site in a ligand specific manner. baNOS forms a five-coordinate, high-spin complex with l-arginine analogs and a six-coordinate low-spin complex with inhibitor imidazole. Studies indicated that the binding of l-arginine, Nω-hydroxy-l-arginine, and imidazole produces various spectroscopic species that closely correspond to the equivalent complexes of mammalian NOS. The values of spectral binding constants further corroborated these results. The overall conservation of the key structural features and the correlation of heme-substrate interactions in baNOS and mammalian NOS, thus, point towards an interesting phenomenon of convergent evolution. Importantly, the NO generated by NOS of mammalian macrophages plays a potent role in antimicrobicidal activity. Because of the existence of high structural and behavioral similarity between mammalian NOS and baNOS, we propose that NO produced by B. anthracis may also have a pivotal pathophysiological role in anthrax infection. Therefore, this first report of characterization of a NOS-like protein from a pathogenic bacterium opens up avenues for further studies in understanding the importance of this protein in pathogenicity. © 2005 Elsevier Inc. All rights reserved.