Extended Data Fig. 1: The nitrous oxide reductase (nos) system.

a, The copper-dependent enzyme N₂OR assembles a composite active site at the homodimer interface, consisting of the mixed-valent [Cu^1.5+:Cu^1.5+] electron transfer centre Cu_A in one monomer, and the unique, tetranuclear Cu_Z cluster with composition [4Cu:2S] in the other monomer. Substrate access and binding occurs between both clusters⁶. b, The Nos system uses the ABC transporter NosFY in conjunction with the periplasmic protein NosD, as well as the dedicated copper chaperone NosL, which was described until now as an outer membrane lipoprotein, whereas the ascribed role of NosD was in sulfur delivery to N₂OR. In addition, the nos operon contains the putative quinol oxidase NosR that requires the NosX protein (in P. stutzeri replaced by AbpE⁵⁷) and may transfer electrons to N₂OR. Alternatively, a soluble cytochrome c₅₅₂ was suggested as physiological electron donor to the enzyme⁵⁸. c, The nos machinery of P. stutzeri is encoded in a gene cluster of the structure nosRZDFYLtatE.