CDP-6-deoxy-L-threo-D-glycero-4-hexulose-3-dehydrase (E1), along with its reductase (E3), catalyzes the unusual C-3 deoxygenation of CDP-6-deoxy-L-threo-D-glycero-4-hexulose to form CDP-3,6-dideoxy-L-threo-D-glycero-4-hexulose in CDP-ascarylose biosynthesis [Chen et al. (1996), Biochemistry, 35, 16412-16420]. This dimeric [2Fe-2S] protein, cloned from the bacteria Yersinia pseudotuberculosis, is currently the only known example of an enzyme that uses a vitamin B6-derived pyridoxamine 5'-phosphate (PMP) cofactor to carry out one-electron chemistry [Agnihotri & Liu (2001), Bioorg. Chem. 29, 234-257]. It also exhibits a [2Fe-2S] cluster-binding motif (C-X57-C-X1-C-X7-C) which has not been observed previously [Agnihotri et al. (2004), Biochemistry, 43, 14265-14274] The recombinant 97.7 kDa dimer was crystallized in the trigonal space group P3(2), with unit-cell parameters a = b = 97.37, c = 142.2 A, alpha = beta = 90, gamma = 120 degrees. A data set has been collected to 1.9 A resolution. A full MAD data set was also collected at the iron absorption edge that diffracted to 2.0 A.