A soluble N-terminal domain of the Escherichia coli adenylyl transferase (ATase) is responsible for deadenylylation activity of the intact enzyme. Previous studies of the deadenylylation activity have involved a fragment, AT-N423 (residues 1 to 423), which was extended by 17 amino acids to give AT-N440. This new domain is truncated at the end of a predicted helix and prior to a Q-linker. The domain was found to be very soluble and stable so that it could be purified to homogeneity and crystallized. This construct has deadenylylation activity that is independent of the low nitrogen status indicator PII-UMP. The crystals belong to space group P3(1)21 or its enantiomorph P3(2)21 with a = b = 116.6 Angstrom and c = 67.6 Angstrom. (C) 2003 Elsevier Inc. All rights reserved.