Selenocysteine lyase is a pyridoxal 5'-phosphate (PLP)-dependent enzyme that catalyzes the exclusive decomposition of L-selenocysteine to L-alanine and elemental selenium. An open reading frame, named csdB, from Escherichia coli encodes a putative protein that is similar to selenocysteine lyase of pig Liver and cysteine desulfurase (NifS) of Azotobacter vinelandii. In this study, the csdB gene was cloned and expressed in E. coli cells. The gene product was a homodimer with the subunit M-r of 44,439, contained 1 mol of PLP as a cofactor per mol of subunit, and catalyzed the release of Se, SO2, and S from L-selenocysteine, L-cysteine sulfinic acid, and L-cysteine, respectively, to yield L-alanine; the reactivity of the substrates decreased in this order. Although the enzyme was not specific for L-selenocysteine, the high specific activity for L-selenocysteine (5.5 units/mg compared with 0.019 units/mg for L-cysteine) supports the view that the enzyme can be regarded as an E. coli counterpart of mammalian selenocysteine lyase. me crystallized CsdB, the csdB gene product, by the hanging drop vapor diffusion method. The crystals were of suitable quality for x-ray crystallography and belonged to the tetragonal space group P4(3)2(1)2 with unit cell dimensions of a = b = 128.1 Angstrom and c = 137.0 Angstrom. Consideration of the Matthews parameter V-m (3.19 Angstrom(3)/Da) accounts for the presence of a single dimer in the crystallographic asymmetric unit. A native diffraction dataset up to 2.8 Angstrom resolution was collected. This is the first crystallographic analysis of a protein of NifS/selenocysteine lyase family.