CRYSTALLIZATION AND PRELIMINARY-X-RAY CRYSTALLOGRAPHIC STUDIES OF ESCHERICHIA-COLI XANTHINE PHOSPHORIBOSYLTRANSFERASE

Xanthine phosphoribosyltransferase (XPRT; EC 2.4.2.22) from Escherichi a coli is a purine salvage enzyme which synthesizes the nucleotides GM P, XMP, and IMP. A mutant C59A, which is more stable than wild-type XP RT while retaining high activity, has been prepared and crystallized t o give three different crystal forms (A, B, and C). Form A crystals ar e orthorhombic (P2(1)2(1)2), with unit cell dimensions a = 59.2 Angstr om, b = 92.9 Angstrom, c = 53.2 Angstrom. Form B crystals are monoclin ic (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 orthorhomb ic form. The selenomethionine derivative was prepared by expressing XP RT in the usual E. coli strain without the need for a methionine auxot roph. Cells were grown in a methionine-deficient medium supplemented w ith selenomethionine which gave > 95% incorporation. Both the wild-typ e and selenomethionine C59A XPRT crystals are isomorphous with C59A fo rm A crystals. (C) 1996 Academic Press, Inc.