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The Rhizobium etli amtB Gene Coding for an NH₄⁺ Transporter Is Down-Regulated Early During Bacteroid Differentiation

During development of root nodules, Rhizobium bacteria differentiate inside the invaded plant cells into N₂-fixing bacteroids. Terminally differentiated bacteroids are unable to grow using the ammonia (NH₃ ) produced therein by the nitrogenase complex. Therefore, the nitrogen assimilation activities of bacteroids, including the ammonium (NH₄ ⁺) uptake activity, are expected to be repressed during symbiosis. By sequence homology the R. etli amtB (ammonium transport) gene was cloned and sequenced. As previously shown for its counterpart in other organisms, the R. etli amtB gene product mediates the transport of NH₄ ⁺. The amtB gene is cotranscribed with the glnK gene (coding for a P_II-like protein) from a nitrogen-regulated σ^54-dependent promoter, which requires the transcriptional activator NtrC. Expression of the glnKamtB operon was found to be activated under nitrogen-limiting, free-living conditions, but down-regulated just when bacteria are released from the infection threads and before transcription of the nitrogenase genes. Our data suggest that the uncoupling between N₂-fixation and NH₃ assimilation observed in symbiosomes is generated by a transcriptional regulatory mechanism(s) beginning with the inactivation of NtrC in younger bacteroids.