. Topological model of NixA in the cytoplasmic membrane. The amino acid sequence of NixA is presented in single-letter code. Boxed regions indicate transmembrane domains. Filled black diamonds indicate the location of PhoA and LacZ reporter fusions by number (from amino terminus) of the last NixA amino acid residue prior to the fusion junction; enzymatic activity of reporter fusions were used to predict the topology (34). Circled residues indicate conserved motifs (among known nickel transporters) in helices II and III, plus six additional transport-critical residues. (Reprinted with permission from reference 34.)