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The Rhizobium GstI Protein Reduces the NH4+ Assimilation Capacity of Rhizobium leguminosarum

July 2001 , Volume 14 , Number  7
Pages  823 - 831

Rosarita Tatè , Luigi Mandrich , Maria R. Spinosa , Anna Riccio , Alessandro Lamberti , Maurizio Iaccarino , and Eduardo J. Patriarca

International Institute of Genetics and Biophysics, CNR, Via G. Marconi 10, 80125 Naples, Italy

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Accepted 12 March 2001.

We show that the protein encoded by the glutamine synthetase translational inhibitor (gstI) gene reduces the NH4+ assimilation capacity of Rhizobium leguminosarum. In this organism, gstI expression is regulated by the ntr system, including the PII protein, as a function of the nitrogen (N) status of the cells. The GstI protein, when expressed from an inducible promoter, inhibits glutamine synthetase II (glnII) expression under all N conditions tested. The induction of gstI affects the growth of a glutamine synthetase I (glnA-) strain and a single amino acid substitution (W48D) results in the complete loss of GstI function. During symbiosis, gstI is expressed in young differentiating symbiosomes (SBs) but not in differentiated N2-fixing SBs. In young SBs, the PII protein modulates the transcription of NtrC-regulated genes such as gstI and glnII. The evidence presented herein strengthens the idea that the endocytosis of bacteria inside the cytoplasm of the host cells is a key step in the regulation of NH4+ metabolism.

Additional keywords: histochemical localization, PCR mutagenesis, posttranscriptional regulation, protein purification, Vicia hirsuta.

© 2001 The American Phytopathological Society