ACTIVITY OF THE TETRAMER AND OCTAMER OF GLUTAMINE-SYNTHETASE ISOFORMSDURING PRIMARY LEAF ONTOGENY OF SUGAR-BEET (BETA-VULGARIS L)

In extracts from the primary leaf blade of sugar beet (Beta vulgaris L .) we separated a chloroplastic isoform (GS 2) of glutamine synthetase (GS, EC 6.3.1.2) and one or two (depending on leaf age) cytosolic iso forms (GS 1a and GS 1b). The latter were prominent in the early (GS 1a ) and late stages of leaf ontogeny (GS 1a and GS 1b), whereas during l eaf maturation GS 2 was the predominantly active GS isoform. The GS 1 isoforms were active exclusively in the octameric state although tetra meric GS 1 protein was detected immunologically. Their activity stayed at a relatively constant level during leaf ontogeny; an increase was observed only in the senescent leaf. The activity of GS 2, however, ch anged drastically during primary leaf ontogeny and was modulated by ch anges in the oligomeric state of the active enzyme. In the early and l ate stages of leaf ontogeny when GS 2 activity was low (lower than tha t of the GS 1 isoforms), GS 2 was active only in the octameric state. In the maturing leaf, when GS 2 activity had reached its maximum level (much higher than that of the GS 1 isoforms), 80% of total GS 2 activ ity was due the activity of the tetrameric form of the enzyme and 20% was due to octameric GS 2. Tetrameric GS 2 was a hetero-tetramer and t hus not the unspecific dissociation product of homo-octameric GS 2. In addition, GS 2 activity was modulated by an activation/inactivation o f the tetrameric GS 2 protein. Due to an activation of the GS 2 tetram er, the activity of tetrameric GS 2 increased during leaf maturation f rom zero level 23-fold compared with that of GS 1a and 18-fold compare d with that of GS 1b. Possible activators of tetrameric GS 2 are thiol -reactive substances. During leaf senescence, GS 2 activity decreased to zero; this decrease was due to an inactivation of the tetrameric GS 2 protein probably caused by oxidation.