Xanthine phosphoribosyltransferase (XPRT; EC 2.4.2.22) from Escherichia coli is a purine salvage enzyme which synthesizes the nucleotides GMP, XMP, and IMP. A mutant C59A, which is more stable than wild-type XPRT while retaining high activity, has been prepared and crystallized to give three different crystal forms (A, B, and C). Form A crystals are orthorhombic (P2(1)2(1)2), with unit cell dimensions a = 59.2 Angstrom, b = 92.9 Angstrom, c = 53.2 Angstrom. Form B crystals are monoclinic (C2) with unit cell dimensions a = 84.4 Angstrom, b = 70.8 Angstrom, c = 54.1 Angstrom and beta = 113.4 degrees, and form C crystals are tetragonal (P4(1)2(1)2 or P4(3)2(1)2) with unit cell dimensions a,b = 94 Angstrom, c = 167.5 Angstrom. Wild-type XPRT and a selenomethionine derivative of C59A XPRT have also been crystallized in the orthorhombic form. The selenomethionine derivative was prepared by expressing XPRT in the usual E. coli strain without the need for a methionine auxotroph. Cells were grown in a methionine-deficient medium supplemented with selenomethionine which gave > 95% incorporation. Both the wild-type and selenomethionine C59A XPRT crystals are isomorphous with C59A form A crystals. (C) 1996 Academic Press, Inc.