Bulletin Spring‧Summer 2001

close Link Between Asparagine Synthetase and Nitrogen Metabolism Prof. Lam Hon-ming of the Department of Biology has since 1998 received a total of over HK$ 1,030,000 from the Research Grants Co u n c i l to study an i mp o r t a n t n i t r o g en me t a b o l ic e n z yme , n ame l y, asparagine synthetase (AS). This enzyme catalyzes asparagine, one major function of wh i ch is to transport and store nitrogen according to the plant's need. It can also reallocate n i t r o g e n d u r i n g s p e c i f i c developmental stages and environmental changes . For e x amp l e, d u r i n g seed development, it helps transport nitrogen to the seeds, and under stress conditions, it stores nitrogen so that it is not wasted. In short, asparagine has a very close relationship with plant growth and development. There are, however, major obstacles in biochemical and physiological studies for the t ho r ough understanding o f the role o f asparagine metabolism. This is ma i n l y because the purification o f plant AS enzymes is difficult. Besides, the production of AS in higher plants is governed by a small gene family (three genes: ASN1,ASN2, and ASNS encoding for three isoenzymes) instea d of one gene. Although their ultimate role is to produce asparagine synthetase, the protein structures they encode and their bioactivities may be different. Hence, each AS enzyme needs to be purified individually for study. Studying Arabidopsis thaliana Using a Molecular-Genetic Approach Prof. Lam used a molecular-genetic approach to explore the functions of each member of the AS gene family. He an d his t e am b e g a n by a n a l y s i n g the p l a n t Arabidopsis thaliana, wh i c h has al l the characteristics o f higher plants. It has a simple hereditary system, and is the firs t plant in the world to have been genetically decoded. The results of the genome project confirmed Prof. Lam's early findings that the AS family has a total of three genes. Prof. Lam believes that the knowledge gained from Arabidopsis thaliana w i l l be applicable to other plants wit h higher agricultural and economic values. The researchers cloned the genes in Arabidopsis thaliana (ASNl, ASN2, and ASN3) and constructed transgenic plants that overproduce these genes to study their physiological roles. Cultivating ASN1 Overexpressing Lines The initial focus of the research was ASN1. The researchers returned the cloned ASN1 gene to the plant to produce ASN1 overexpressing lines, in order to observe nitrogen metabolism i n such lines. The y Arabidopsis thaliana — the model plant Asparagine Synthetase Genes: Their Roles in Plant Growth and Development 31