DEVELOPMENTAL-CHANGES IN GLUTAMINE-SYNTHETASE ISOFORMS IN SAMBUCUS-NIGRA AND TRIENTALIS-EUROPAEA

It has been suggested that cytosolic glutamine synthetase re-assimilat es ammonium derived from protein breakdown during senescence, to produ ce glutamine for transport. This paper is an initial report of the act ivities, polypeptide composition and molecular biology of glutamine sy nthetase (GS) (EC 6.3.1.2) isoforms in two very different deciduous pe rennials, Sambucus nigra and Trientalis europaea. ion-exchange chromat ography and SDS-PAGE immunoblotting revealed profound temporal and spa tial differences in GS activity in the two species. In S. nigra the ch loroplastic (GS2) isoform, which was composed of 45 kDa polypeptides, predominated early in the year but declined during senescence, whereas the cytosolic (GSI) isoform, which was composed of a 38/40 kDa double r, was not detected in spring but increased in activity to a peak in a utumn. In T. europaea two GS activities in the leaf contributed the sa me proportion of the total GS activity throughout the growing season, whereas the mainly cytosolic activity in the rhizome increased 6-fold when the shoot was senescing. The GSI peak in T. europaea leaf contain ed both chloroplastic and cytosolic activities composed of polypeptide s of the size expected for those isoforms (44 and 40 kDa respectively) . However, the GS2 peak, which was shown to contain a second chloropla stic GS activity, contained a polypeptide of the size normally associa ted with cytosolic GS (39 kDa). This is the first known report of a sp ecies with two chloroplastic CS isoforms, and the first report of a ch loroplastic isoform being composed of such small polypeptides.