The rhizobium GstI protein reduces the NH4+ assimilation capacity of Rhizobium leguminosarum

We show that the protein encoded by the glutamine synthetase translational inhibitor (gstI) gene reduces the NH4+ assimilation capacity of Rhizobium l eguminosarum. In this organism, gstI expression is regulated by the ntr sys tem, including the P-II protein, as a function of the nitrogen (N) status o f the cells. The GstI protein, when expressed from an inducible promoter, i nhibits glutamine synthetase II (glnII) expression under all N conditions t ested. The induction of gstI affects the growth of a glutamine synthetase I (glnA(-)) strain and a single amino acid substitution (W48D) results in th e complete loss of GstI function. During symbiosis, gstI is expressed in yo ung differentiating symbiosomes (SBs) but not in differentiated N-2-fixing SBs. In young SBs, the P-II protein modulates the transcription of NtrC-reg ulated genes such as gstI and glnII. The evidence presented herein strength ens the idea that the endocytosis of bacteria inside the cytoplasm of the h ost cells is a key step in the regulation of NH4+ metabolism.