GlnD is a bifunctional uridylyltransferase/uridylyl -removing enzyme that has a central role in the general nitrogen regulatory system NTR. In enterobacteria, GlnD uridylylates the PII proteins GInB and GlnK under low levels of fixed nitrogen or ammonium. Under high ammonium levels, GlnD removes UMP from these proteins (deuridylylation). The PII proteins are signal transduction elements that integrate the signals of nitrogen, carbon and energy, and transduce this information to proteins involved in nitrogen metabolism. In Herbaspirillurn seropedicae, an endophytic diazotroph isolated from grasses, several genes coding for proteins involved in nitrogen metabolism have been identified and cloned, including gInB, glnK and glnD. In this work, the GlnB, GlnK and GlnD proteins of H. seropedicae were overexpressed in their native forms, purified and used to reconstitute the uridylylation system in vitro. The results show that H. seropedicae GlnD uridylylates GlnB and GlnK trimers producing the forms PII (UMP)(1), PII (UMP)(2) and PII (UMP)(3), in a reaction that requires 2-oxoglutarate and ATP, and is inhibited by glutamine. The quantification of these PH forms indicates that GlnB was more efficiently uridylylated than GlnK in the system used. (C) 2007 Elsevier Inc. All rights reserved.