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Plant Physiol, May 2001, Vol. 126, pp. 109-121
Constitutive Overexpression of Cytosolic Glutamine Synthetase
(GS1) Gene in Transgenic Alfalfa Demonstrates That
GS1 May Be Regulated at the Level of RNA Stability and
Protein Turnover1
Jose Luis
Ortega,
Stephen J.
Temple,2 and
Champa
Sengupta-Gopalan*
Agronomy and Horticulture Department, New Mexico State University,
Las Cruces, New Mexico 88003
Glutamine synthetase (GS) catalyzes the ATP-dependent condensation
of NH4+ with glutanate to yield glutamine. Gene
constructs consisting of the cauliflower mosaic virus (CaMV) 35S
promoter driving a cytosolic isoform of GS (GS1) 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  -glucuronidase activity could be detected in
nodules of plants containing the CaMV 35S promoter--glucuronidase
gene construct, suggesting that the transcript for the GS1
transgene is not stable in the root nodules. Leaves of alfalfa plants
with the CaMV 35S promoter-GS1 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
GS1 transcripts when fed with high levels of
NO3 . However, no increase in GS activity or
polypeptide level was detected in the leaves of transgenic plants. The
results suggest that GS1 is regulated at the level of RNA
stability and protein turnover.
1
This work was supported by the National Science
Foundation (grant no. IBN-92201 42), by the National Institutes of
Health (grant no. GMO-8136-26), and by the Agricultural Experiment
Station at New Mexico State University. J.L.O. was a recipient of a
1-year fellowship from Dirección General de Asuntoas para el
Personal Académico, National University of Mexico.
2
Present address: Forage Genetics, N5292 South Gills
Coulee Road, West Salem, WI 54669.
*
Corresponding author; e-mail csgopala{at}nmsu.edu; fax
505- 646-6041.
© 2001 American Society of Plant Physiologists
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