Ben-Tzvi Tzchori I, Perl A, Galili G
Department of Plant Genetics, Weizmann Institute of Science, Rehovot, Israel.
Plant Mol Biol. 1996 Nov;32(4):727-34. doi: 10.1007/BF00020213.
To study the regulation of lysine and threonine metabolism in plants, we have transformed Arabidopsis thaliana with chimeric genes encoding the two bacterial enzymes dihydrodipicolinate synthase (DHPS) and aspartate kinase (AK). These bacterial enzymes are much less sensitive to feedback inhibition by lysine and threonine than their plant counterparts. Transgenic plants expressing the bacterial DHPS overproduced lysine, but lysine levels were quite variable within and between transgenic genotypes and there was no direct correlation between the levels of free lysine and the activity of DHPS. The most lysine-overproducing plants also exhibited abnormal phenotypes. However, these phenotypes were detected only at early stages of plant growth, while at later stages, new buds emerged that looked completely normal and set seeds. Wild-type plants exhibited relatively high levels of free threonine, suggesting that in Arabidopsis AK regulation may be more relaxed than in other plants. This was also supported by the fact that expression of the bacterial AK did not cause any dramatic elevation in this amino acid. Yet, the relaxed regulation of threonine synthesis in Arabidopsis was not simply due to a reduced sensitivity of the endogenous AK to feedback inhibition by lysine and threonine because growth of wild-type plants, but not of transgenic plants expressing the bacterial AK, was arrested in media containing these two amino acids. The present results, combined with previous studies from our laboratory, suggest that the regulation of lysine and threonine metabolism is highly variable among plant species and is subject to complex biochemical, physiological and environmental controls. The suitability of these transgenic Arabidopsis plants for molecular and genetic dissection of lysine and threonine metabolism is also discussed.
为了研究植物中赖氨酸和苏氨酸的代谢调控，我们用编码两种细菌酶——二氢吡啶二羧酸合酶（DHPS）和天冬氨酸激酶（AK）的嵌合基因转化了拟南芥。这些细菌酶对赖氨酸和苏氨酸反馈抑制的敏感性远低于其植物对应物。表达细菌DHPS的转基因植物过量产生赖氨酸，但赖氨酸水平在转基因基因型内和基因型间差异很大，游离赖氨酸水平与DHPS活性之间没有直接相关性。赖氨酸过量产生最多的植物也表现出异常表型。然而，这些表型仅在植物生长早期被检测到，而在后期，长出的新芽看起来完全正常并结出种子。野生型植物表现出相对较高水平的游离苏氨酸，这表明在拟南芥中AK调控可能比其他植物更为宽松。细菌AK的表达并未导致该氨基酸显著升高这一事实也支持了这一点。然而，拟南芥中苏氨酸合成调控的宽松并非仅仅由于内源性AK对赖氨酸和苏氨酸反馈抑制的敏感性降低，因为在含有这两种氨基酸的培养基中，野生型植物的生长受到抑制，而表达细菌AK的转基因植物则不受影响。目前的结果与我们实验室之前的研究相结合，表明赖氨酸和苏氨酸代谢调控在植物物种间差异很大，且受到复杂的生化、生理和环境控制。还讨论了这些转基因拟南芥植物在赖氨酸和苏氨酸代谢的分子和遗传剖析方面的适用性。
