The Rhizobium leguminosarum glnB gene is down-regulated during symbiosis

Symbiotic nitrogen fixation involves the development, on the legume plant r oot, of specialised organs called nodules, within which plant photosynthate s are exchanged for combined nitrogen of bacterial origin. The glnB gene en codes a signal transduction protein (PII) which is a component of the bacte rial nitrogen regulation (Ntr) system and an essential regulator of ammoniu m assimilation. We demonstrate that in Rhizobium leguminosarum the glnB pro moter is strongly regulated by nitrogen and NtrC, but still shows a signifi cant level of activity in conditions of nitrogen excess. Expression of gene s involved in nitrogen assimilation has been shown to be absent in nitrogen -fixing bacteroids, and, in agreement with this, we find that the glnB prom oter is down-regulated during bacteroid differentiation at a time coinciden t with the arrest of bacterial division in the nodule. This pattern is comm on to other bacterial genes involved in nitrogen assimilation and it is not eworthy that the zone where the glnB promoter is active is coincident with the region in which NtrC is expressed.