<p>The members of this entry from <taxon tax_id="562">Escherichia coli</taxon>, <taxon tax_id="303">Pseudomonas putida</taxon>, and <taxon tax_id="29459">Brucella melitensis</taxon>, are found associated with a nickel ABC transporter operon that acts to import nickel for use as a cofactor in urease or hydrogenase. These proteins, with characterised nickel-binding and DNA-binding domains, act as nickel-responsive transcriptional regulators. In the larger family of full-length homologues, most others both lack proximity to the nickel ABC transporter operon and form a separate clade. Several of the homologues not in this entry have been shown to bind nickel, copper, or both, and to regulate genes in response to nickel.</p>