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The Rhizobium leguminosarum bv. viciae glnD gene, encoding a uridylyltransferase/uridylyl-removing enzyme, is expressed in the root nodule but is not essential for nitrogen fixation

Ökologie des Bodens, Botanisches Institut, RWTH Aachen, Worringerweg 1, 52056 Aachen, Germany¹
International Institute of Genetics and Biophysics – CNR, Via Marconi 12, 80125 Napoli, Italy²

Author for correspondence: Ursula B. Priefer. Tel: +49 241 80 6644. Fax: +49 241 8888 637. e-mail: priefer{at}bio1.rwth-aachen.de

A Rhizobium leguminosarum bv. viciae VF39 gene (glnD) encoding the uridylyltransferase/uridylyl-removing enzyme, which constitutes the sensory component of the nitrogen regulation (ntr) system, was identified, cloned and characterized. The deduced amino acid sequence contains the conserved active site motif of the nucleotidyltransferase superfamily and is highly homologous to the glnD gene products of other bacterial species. Downstream of the VF39 glnD resides an open reading frame with similarity to the Salmonella typhimurium virulence factor gene mviN. Mutation of the glnD gene abolished the ability to use nitrate as a sole nitrogen source but not glutamine. In addition, neither uridylylation of P_II nor induction of the ntr-regulated glnII gene (encoding glutamine synthetase II) under ammonium deficiency could be observed in mutant strains. This strongly suggests that glnD mutants harbour a permanently deuridylylated P_II protein and as a consequence are unable to activate transcription from NtrC-dependent promoters. The glnD gene itself is expressed constitutively, irrespective of the nitrogen content of the medium. A functional GlnD protein is not essential for nitrogen fixation in R. leguminosarum bv. viciae, but in situ detection of glnD expression in the symbiotic and infection zone of the root nodule and quantitative measurements suggest that at least part of the ntr system functions in symbiosis. The results also indicate that the N-terminal part of GlnD is essential for the cell, as deletions in the 5'-region of the gene appear to be lethal and mutations possibly affecting the expression of the first half of the protein have a significant effect on the vitality of the mutant strain.

Keywords: nitrogen regulation, uridylyltransferase, P_II uridylylation, glutamine synthetase, mviN

Abbreviations: GS, glutamine synthetase

^a Present address: Lehrstuhl für Genetik, Universität Bielefeld, Postfach 10 01 31, 33501 Bielefeld, Germany.

The GenBank accession number for the sequence reported in this paper is AF155830.

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