PLANT PHYSIOLOGY , Vol 112, Issue 3 987-996, Copyright © 1996 by American Society of Plant Biologists
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GENE REGULATION AND MOLECULAR GENETICS |
Isolation and Characterization of Glutamine Synthetase Genes in Chlamydomonas reinhardtii
Q. Chen and C. D. Silflow
Department of Genetics and Cell Biology (Q.C., C.D.S.), Plant Molecular Genetics Institute (Q.C., C.D.S.), and Department of Plant Biology (C.D.S.), University of Minnesota, St. Paul, Minnesota 55108
To elucidate the role of glutamine synthetase (GS) in nitrogen assimilation
in the green alga Chlamydomonas reinhardtii we used maize GS1 (the
cytosolic form) and GS2 (the chloroplastic form) cDNAs as hybridization
probes to isolate C. reinhardtii cDNA clones. The amino acid sequences
derived from the C. reinhardtii clones have extensive homology with GS
enzymes from higher plants. A putative amino-terminal transit peptide
encoded by the GS2 cDNA suggests that the protein localizes to the
chloroplast. Genomic DNA blot analysis indicated that GS1 is encoded by a
single gene, whereas two genomic fragments hybridized to the GS2 cDNA
probe. All GS2 cDNA clones corresponded to only one of the two GS2 genomic
sequences. We provide evidence that ammonium, nitrate, and light regulate
GS transcript accumulation in green algae. Our results indicate that the
level of GS1 transcripts is repressed by ammonium but induced by nitrate.
The level of GS2 transcripts is not affected by ammonium or nitrate.
Expression of both GS1 and GS2 genes is regulated by light, but perhaps
through different mechanisms. Unlike in higher plants, no decreased level
of GS2 transcripts was detected when cells were grown under conditions that
repress photorespiration. Analysis of GS transcript levels in mutants with
defects in the nitrate assimilation pathway show that nitrate assimilation
and ammonium assimilation are regulated independently.
