Chen Q, Silflow C D
Department of Genetics and Cell Biology, University of Minnesota, St. Paul 55108, USA.
Plant Physiol. 1996 Nov;112(3):987-96. doi: 10.1104/pp.112.3.987.
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.
为阐明谷氨酰胺合成酶（GS）在莱茵衣藻氮同化中的作用，我们使用玉米GS1（胞质形式）和GS2（叶绿体形式）的cDNA作为杂交探针来分离莱茵衣藻的cDNA克隆。从莱茵衣藻克隆中推导的氨基酸序列与高等植物的GS酶具有广泛的同源性。GS2 cDNA编码的一个假定的氨基端转运肽表明该蛋白定位于叶绿体。基因组DNA印迹分析表明，GS1由单个基因编码，而两个基因组片段与GS2 cDNA探针杂交。所有GS2 cDNA克隆仅对应于两个GS2基因组序列中的一个。我们提供的证据表明，铵、硝酸盐和光可调节绿藻中GS转录本的积累。我们的结果表明，GS1转录本的水平受铵抑制，但受硝酸盐诱导。GS2转录本的水平不受铵或硝酸盐的影响。GS1和GS2基因的表达均受光调节，但可能通过不同的机制。与高等植物不同，当细胞在抑制光呼吸的条件下生长时，未检测到GS2转录本水平的降低。对硝酸盐同化途径有缺陷的突变体中GS转录本水平的分析表明，硝酸盐同化和铵同化是独立调节的。
