TOTAL GLUTAMINE-SYNTHETASE ACTIVITY DURING SOYBEAN NODULE DEVELOPMENTIS CONTROLLED AT THE LEVEL OF TRANSCRIPTION AND HOLOPROTEIN TURNOVER

Gln synthetase (CS) catalyzes the ATP-dependent condensation of ammoni a with glutamate to yield Gln. In higher plants GS is an octameric enz yme and the subunits are encoded by members of a small multigene famil y. In soybeans (Glycine max), following the onset of N-2 fixation ther e is a dramatic increase in GS activity in the root nodules. GS activi ty staining of native polyacrylamide gels containing nodule and root e xtracts showed a common band of activity (GSrs). The nodules also cont ained a slower-migrating, broad band of enzyme activity (GSns). The GS ns activity band is a complex of many isozymes made up of different pr oportions of two kinds of GS subunits: GSr and GSn. Root nodules forme d following inoculation with an Nif(-) strain of Bradyrhizobium japoni cum showed the presence of GS isoenzymes (GSns1) with low enzyme activ ity, which migrated more slowly than GSns. Gsns1 is most likely made u p predominantly of GSn subunits. Our data suggest that, whereas the cl ass I GS genes encoding the GSr subunits are regulated by the availabi lity of NH3, the class II GS genes coding for the GSn subunits are dev elopmentally regulated. Furthermore, we have demonstrated that the GSn s1 isozymes in the Nif(-) nodules are relatively more labile. Our over all conclusion is that GSns activity in soybean nodules is regulated b y N-2 fixation both at the level of transcription and at the level of holoprotein stability.