This domain is found on many type IV pili methyl-accepting chemotaxis transducer proteins [1] where there is also a HAMP signature towards the C-terminus. It is a monomodular four-helix bundle and recognises nitrate and nitrite (Matilla et al. FEMS Microbiology Reviews, fuab043, 45, 2021, 1. https://doi.org/10.1093/femsre/fuab043).

1: Structure of Clostridium difficile PilJ exhibits unprecedented divergence from known type IV pilins. Piepenbrink KH, Maldarelli GA, de la Pena CF, Mulvey GL, Snyder GA, De Masi L, von Rosenvinge EC, Gunther S, Armstrong GD, Donnenberg MS, Sundberg EJ; J Biol Chem. 2014;289:4334-4345. PMID:24362261