Isoforms of glutamine synthetase in Panicum species having C3, C4, and intermediate photosynthetic pathways

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Abstract
The number, relative activity, and immunological nature of glutamine synthetase (GS) isoforms were studied in three species of Panicum, each differing in the nature of their photosynthetic system: P. bisulcatum, a C3 plant; P. milioides, a C3–C4 intermediate; and P. miliaceum, a C4 plant. Ion-exchange chromatography of leaf extracts for all three species revealed a peak of GS activity eluting at 0.23–0.32 M NaCl. These peaks had similar elution characteristics to GS isolated from chloroplasts of barley. Also, as in the barley chloroplastic GS, they were poorly recognized by antibodies from barley cytosolic GS (GS1). These peaks were therefore identified as GS2 and were thought to be chloroplastic in origin. A second peak of GS activity eluting at 0.18 M NaCl was found to account for 0, 21, and 45% of the total GS activity in the C3, C3–C4 intermediate, and C4 leaves, respectively. It had similar elution characteristics to the cytosolic GS from barley and was readily bound by the antibodies made against barley cytosolic GS (GS1). This enzyme, identified as GS1, was thought to be cytosolic in origin. The results demonstrate that even within a single genus, the GS isoform composition is closely related to the type of photosynthetic pathway present within the leaf tissue.