Isozymes of Glutamine Synthetase in Phaseolus vulgaris L. and Phaseolus lunatus L. Root Nodules

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Abstract
The glutamine synthetase (GS) isozymes in the plant fraction of nodule extracts from 62 cultivars of Phaseolus vulgaris L. and one cultivar of Phaseolus lunatus L. were analyzed by polyacrylamide gel electrophoresis. All P. vulgaris nodule extracts displayed two GS activity bands: a nodule-specific band (GSnl) and a band (GSn2) similar to the single band (GSr) present in root extracts. In nodule extracts of P. lunatus, the GSn1 band was detected, but the GSn2 band was barely detectable. In contrast to P. vulgaris, the GSn2 band and the GSr band of P. lunatus appeared to be different. The electrophoretic mobility of the GSn1 band in P. vulgaris was governed by both the plant cultivar and the development stage of the nodule. In nodule extracts of P. vulgaris and P. lunatus, the zone of GSn1 activity coincided with six to nine distinct protein bands as revealed after treatment of gels, which had previously been stained for GS activity, with Coomassie blue. All these protein bands were shown to consist of polypeptides of identical molecular weight (approximately 47,000 daltons) by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Our results indicte that P. vulgaris continously generates isozymes of GSn1 of increasing electrophoretic mobility during the course of nodule development.