Immunological studies of glutamine synthetase inFrankia-Alnus incanasymbioses

Abstract

We have investigated the presence and form of glutamine synthetase (GS) in Frankia vesicle cluster preparations of two actively nitrogen-fixing Frankia-Alnus incana root-nodule symbioses and in cultured Frankia sp. strain CpI1 (HFP070101). The symbioses contained Frankia CpI1 or the local source of Frankia. We used Western-blot analysis with antisera raised against three types of GS. In symbiotic Frankia GS protein was not detected at a significant level when either antisera against Rhodospirillum rubrum GS or antisera against Rhizobium meliloti GSII were used. In cultured Frankia CpI1 GSI was detected both when grown with NH4+ or N2 as nitrogen source, and GSII was detected when grown on N2. Antiserum raised against the nodule-specific GSn1 of Phaseolus vulgaris crossreacted with a 43-kDa polypeptide corresponding to plant GS in root-nodule extracts from Alnus, and with a 41-kDa polypeptide corresponding to GSII in cultured Frankia CpI1 grown on N2. We conclude that both GSI and GSII are repressed in symbiotic Frankia and that NH4+ produced through nitrogen fixation is assimilated by the plant in Frankia-Alnus incana symbioses. It thus appears that vesicle formation, synthesis of nitrogenase and synthesis of GS are separately regulated in symbiotic Frankia and that the plant has to supply symbiotic Frankia with organic nitrogen in some form in addition to the carbon.