This conserved region includes the FMN-binding site of the NqrC protein [1] as well as the NosR and NirI regulatory proteins. This domain is post-translationally flavinylated that may facilitate electron transfer, and thus, resembles multiheme cytochromes [3].

1: Expression and mutagenesis of the NqrC subunit of the NQR respiratory Na(+) pump from Vibrio cholerae with covalently attached FMN. Barquera B, Hase CC, Gennis RB; FEBS Lett 2001;492:45-49. PMID:11248234

2: New Knowledge from Old: In silico discovery of novel protein domains in Streptomyces coelicolor. Yeats C, Bentley S, Bateman A; BMC Microbiol 2003;3:3-3. PMID:12625841

3: Post-translational flavinylation is associated with diverse extracytosolic redox functionalities throughout bacterial life. Meheust R, Huang S, Rivera-Lugo R, Banfield JF, Light SH; Elife. 2021; [Epub ahead of print] PMID:34032212