Constitutive overexpression of cytosolic glutamine synthetase (GS(1)) genein 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 NH4with glutanate to yield glutamine. Gene constructs consisting of the caulif lower mosaic virus (CaMV) 35S promoter driving a cytosolic isoform of GS (G S(1)) gene have been introduced into alfalfa (Medicago sativa). Although tr anscripts for the transgene were shown to accumulate to high levels in the leaves, they were undetectable in the nodules. However, significant amounts of beta -glucuronidase activity could be detected in nodules of plants con taining the CaMV 35S promoter-beta -glucuronidase gene construct, suggestin g that the transcript for the GS(1) transgene is not stable in the root nod ules. 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 o f GS(1) transcripts when fed with high levels of NO3-. However, no increase in GS activity or polypeptide level was detected in the leaves of transgen ic plants. The results suggest that GS(1) is regulated at the level of RNA stability and protein turnover.