Azospirillum brasilense is a diazotroph which associates with important agricultural crops. The nitrogen fixation process in this organism is highly regulated by ammonium and oxygen, and involves several proteins including the two I'll-like proteins, GlnB and GlnZ. Although these proteins are structurally very similar, they play different roles in the control of nitrogen fixation. In this work, we describe the expression, purification, and uridylylation of the GlnZ protein of A. brasilense strain FP2. The amplified glnZ gene was sub-cloned and expressed as a His-tagged fusion protein. After purification, we obtained 30-40 mg of purified GlnZ per liter of culture. This protein was purified to 99% purity and assayed for in vitro uridylylation using a partially purified Escherichia coli GlnD as a source of uridylylyl-transferase activity. Analyses of the uridylylation reactions in non-denaturing and denaturing polyacrylamide gel electrophoresis showed that up to 74% of GInZ monomers were modified after 30 min reaction. This covalent modification is strictly dependent on ATP and 2-ketoglutarate, while glutamine acts as an inhibitor and promotes deuridylylation. (C) 2003 Elsevier Inc. All rights reserved.