Escherichia coli flavorubredoxin is a member of the family of the A-type flavoproteins, which are built by two core domains: a metallo-beta-lactamase-like domain, at the N-terminal region, harboring a non-heme di-iron site, and a flavodoxin-like domain, containing one FMN moiety. The enzyme from E. coli has an extra module at the C terminus, containing a rubredoxin-like center. The A-type flavoproteins are widespread among strict and facultative anaerobes, as deduced from the analysis of the complete prokaryotic genomes. In this report we showed that the recombinant enzyme purified from E. coli has nitric-oxide reductase activity with a turnover number of approximately 15 mol of NO.mol enzyme(-1).s(-1), which was well within the range of those determined for the canonical heme b(3)-Fe(B) containing nitric-oxide reductases (e.g. approximately 10-50 mol NO.mol enzyme(-1).s(-1) for the Paracoccus denitrificans NOR). Furthermore, it was shown that the activity was due to the A-type flavoprotein core, as the rubredoxin domain alone exhibited no activity. Thus, a novel family of prokaryotic NO reductases, with a non-heme di-iron site as the catalytic center, was established