Two structures of phosphoribosyl isomerase A (PriA) from Streptomyces coelicolor, involved in both histidine and tryptophan biosynthesis, were solved at 1.8 angstrom resolution. A closed conformer was obtained, which represents the first complete structure of PriA, revealing hitherto unnoticed molecular interactions and the occurrence of conformational changes. Inspection of these conformers, including ligand-docking simulations, allowed identification of residues involved in substrate recognition, chemical catalysis and conformational changes. These predictions were validated by mutagenesis and functional analysis. Arg(19) and Ser(81) were shown to play critical roles within the carboxyl and amino phosphate-binding sites, respectively; the catalytic residues Asp(11) and Asp(130) are responsible for both activities; and Thr(166) and Asp(171), which make an unusual contact, are likely to elicit the conformational changes needed for adopting the active site architectures. This represents the first report of the structure/function relationship of this (beta alpha)(8)-isomerase. (c) 2007 Elsevier Inc. All rights reserved.