Ammonia Regulation of Glutamine Synthetase in Rhizobium sp. ANU289

Rhizobium sp. ANU289 lacked glutamate dehydrogenase (GDH) but grew rapidly on ammonia in a well-buffered medium. The properties of glutamine synthetase I (GSI) differed from those of GS from enteric bacteria in a manner which appeared to compensate for the lack of GDH. Adenylylated and unadenylylated GSI had the same biosynthetic activity, and there was also no detectable difference between the two forms with respect to affinities for substrates. GSII, but not GSI, was repressed by growth on ammonia, further suggesting that GSI is important in ammonia assimilation in the presence of high ammonia concentrations. Methylamine was found to exert similar regulatory effects to ammonia and was therefore used to determine the extent to which utilization of various nitrogen sources was controlled by ammonia in strain ANU289.