Constitutive Overexpression of Cytosolic Glutamine Synthetase (GS(1)) Gene in Transgenic Alfalfa Demonstrates That GS(1) May Be Regulated at the Level of RNA Stability and Protein Turnover

Glutamine synthetase (GS) catalyzes the ATP-dependent condensation of NH(4)(+) with glutanate to yield glutamine. Gene constructs consisting of the cauliflower mosaic virus (CaMV) 35S promoter driving a cytosolic isoform of GS (GS(1)) gene have been introduced into alfalfa (Medicago sativa). Although transcripts for the transgene were shown to accumulate to high levels in the leaves, they were undetectable in the nodules. However, significant amounts of β-glucuronidase activity could be detected in nodules of plants containing the CaMV 35S promoter-β-glucuronidase gene construct, suggesting that the transcript for the GS(1) transgene is not stable in the root nodules. Leaves of alfalfa plants with the CaMV 35S promoter-GS(1) gene showed high levels of accumulation of the transcript for the transgene when grown under low-nitrogen conditions and showed a significant drop in the level of GS(1) transcripts when fed with high levels of NO(3)(−). However, no increase in GS activity or polypeptide level was detected in the leaves of transgenic plants. The results suggest that GS(1) is regulated at the level of RNA stability and protein turnover.