The C-terminal segment of copper-containing nitrite reductase from Achromobacter cycloclastes (AcNiR) has been found essential for maintaining both the quaternary structure and the enzyme activity of AcNiR. C-terminal despentapeptide AcNiR (NiRc-5) and desundecapeptide AcNiR (NiRc-11) are two important truncated mutants whose activities and stability ha,c been affected by residue deletion. In this study, the two mutants ",ere crystallized using the hanging drop vapor diffusion method. Crystals of NiRc-5 obtained at pH 5.0 and 6.2 both belonged to the P-2(1)2(1)2(1) space group with unit cell parameters a = 99.0 Angstrom, b = 117.4 Angstrom, c = 122.8 Angstrom (pH 5.0) and a = 98.9 Angstrom, b = 117.7 Angstrom, c = 123.0 Angstrom (pH 6.2). NiRc-11 was crystallized in two crystal forms: the tetragonal form belonged to the space group P4(1) with a = b 96.0 Angstrom and c = 146.6 Angstrom the monoclinic form belonged to the space group P2(1) with a = 86.0 Angstrom, h = 110.1 Angstrom, c = 122.7 Angstrom, and beta = 101.9degrees. The crystallizing behaviors of the two mutants differed from that of the native enzyme. Such change in combination with residue deletion is also discussed here. (C) 2002 Elsevier Science (USA). All rights reserved.