Variable selective pressures (ω) along NAT protein sequence. The posterior mean of ω was estimated for each of the 290 amino acid residues by the codeml program (a) in the vertebrate dataset (n = 77 coding sequences), (b) in the mammalian dataset (n = 55 coding sequences), (c) in the primate dataset (n = 43 coding sequences), (d) in the set of 19 simian NAT2 coding sequences. A graphical representation of the three-domain structure of human NAT1 and NAT2 proteins is shown at the bottom. Residues 1–83 in the N-terminus form domain I, mainly consisting of α-helices; residues 84–192 form domain II, mainly consisting of β-strands, followed by inter-domain comprising residues 193–229, and residues 230–290 consisting of both α-helix and β-strands form domain III. The amino and carboxyl termini are labeled as NT and CT, respectively. The 17-residue insertion and carboxyl-terminal tail of human NATs, which are lacking in the structures of prokaryotic NATs, are highlighted with thick lines. Residues of the catalytic triad are highlighted with stars, residues interacting with CoA are highlighted with black triangles, and residues involved in substrate binding are highlighted with white triangles.