THE RHIZOBIUM-ETLI AMTB GENE CODING FOR AN NH4-REGULATED EARLY DURINGBACTEROID DIFFERENTIATION( TRANSPORTER IS DOWN)

During development of root nodules, Rhizobium bacteria differentiate i nside the invaded plant cells into N-2-fixing bacteroids. Terminally d ifferentiated bacteroids are unable to grow using the ammonia (NH3) pr oduced therein by the nitrogenase complex. Therefore, the nitrogen ass imilation activities of bacteroids, including the ammonium (NH4+) upta ke activity, are expected to be repressed during symbiosis. By sequenc e homology the R. etli amtB (ammonium transport) gene was cloned and s equenced. 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 prot ein) from a nitrogen-regulated sigma(54)-dependent promoter, which req uires the transcriptional activator NtrC. Expression of the glnKamtB o peron was found to be activated under nitrogen-limiting, free-living c onditions, 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 N-2-fixation and NH3 assi milation observed in symbiosomes is generated by a transcriptional reg ulatory mechanism(s) beginning with the inactivation of NtrC in younge r bacteroids.