Glutamine Synthetase (GS): A Key Enzyme for Nitrogen Assimilation in The Macroalga Gracilariopsis lemaneiformis (Rhodophyta)

This study aimed to address the importance of glutamine synthetase II ( GSII ) during nitrogen assimilation in macroalga Gracilariopsis lemaneiformis . The cDNA full‐length sequence of the three gl GSII genes was revealed to have the 5′ m 7 G cap, 5′‐untranslated region, open reading frame ( ORF ), 3′‐untranslated region, and a 3′ poly (A) tail. The three gl GSII s were classified into plastid gl GS 2 and cytosolic gl GS 1‐1 and gl GS 1‐2, having conserved GSII domains but different cDNA sequences. The complicated 5′ end flanking region indicates complex function of gl GS genes. gl GS 1 genes were significantly up‐regulated under the different NH 4 + : NO 3 − ratio (i.e., 40:10, 25:25, 10:40, and 0:50) except gl GS 2 which dramatically up‐regulated under the low NH 4 + : NO 3 − ratio (i.e., 10:40 and 0:50) during different cultivation times. These different expression patterns perhaps are due to the different biological roles of GS 1 and GS 2 in the gene family. Furthermore, hypothetical working model of nitrogen assimilation pathway exhibiting the role of gl GS 1 and gl GS 2 is proposed. Finally, gl GS 2 was expressed in Escherichia coli BL 21 ( DE 3), and the optimal conditions for culture (15°C, overnight), purification (500 mM imidazole washing), and activity ( pH 7.4, 37°C) were established. This study lays a very important foundation for exploring the role of GS in nitrogen assimilation in algae and plants.