Taté R, Riccio A, Merrick M, Patriarca E J
International Institute of Genetics and Biophysics, CNR, Naples, Italy.
Mol Plant Microbe Interact. 1998 Mar;11(3):188-98. doi: 10.1094/MPMI.1998.11.3.188.
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 PII-like protein) from a nitrogen-regulated sigma 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.
在根瘤发育过程中，根瘤菌在被侵染的植物细胞内分化为固氮类菌体。终末分化的类菌体无法利用固氮酶复合物在其中产生的氨（NH₃）进行生长。因此，预计类菌体的氮同化活性，包括铵（NH₄⁺）摄取活性，在共生过程中会受到抑制。通过序列同源性克隆并测序了费氏中华根瘤菌amtB（铵转运）基因。如先前在其他生物体中所显示的那样，费氏中华根瘤菌amtB基因产物介导NH₄⁺的转运。amtB基因与glnK基因（编码一种类PII蛋白）从一个受氮调节的依赖于σ⁵⁴的启动子共转录，该启动子需要转录激活因子NtrC。发现glnKamtB操纵子的表达在氮限制的自由生活条件下被激活，但就在细菌从感染丝中释放出来时以及在固氮酶基因转录之前被下调。我们的数据表明，在共生体中观察到的N₂固定与NH₃同化之间的解偶联是由一种转录调控机制产生的，该机制始于较年轻类菌体中NtrC的失活。
