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.