Structural adaptation to selective pressure for altered ligand specificityin the Pseudomonas aeruginosa amide receptor, AmiC

The AmiC protein in Pseudomonas aeruginosa is the negative regulator and li gand receptor for an amide-inducible aliphatic amidase operon, In the wild- type PAC1 strain, amidase expression is induced by acetamide or lactamide, but not by butyramide, A mutant strain of P.aeruginosa, PAC181, was selecte d for its sensitivity to induction by butyramide. The molecular basis for t he butyramide inducible phenotype of P.aeruginosa PAC181 has now been deter mined, and results from a Thr-->Asn mutation at position 106 in PAC181-AmiC . In the wild-type PAC1-AmiC protein this residue forms part of the side wa ll of the amide-binding pocket but does not interact with the acetamide lig and directly. In the crystal structure of PAC181-AmiC complexed with butyra mide, the Thr-->Asn mutation increases the size of the ligand binding site such that the mutant protein is able to close into its 'on' configuration e ven in the presence of butyramide, Although the mutation allows butyramide to be recognized as an inducer of amidase expression, the mutation is struc turally sub-optimal, and produces a significant decrease in the stability o f the mutant protein.