Wang X, Stumpf D K, Larkins B A
Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721, USA.
Plant Physiol. 2001 Apr;125(4):1778-87. doi: 10.1104/pp.125.4.1778.
The maize (Zea mays) Oh545o2 inbred accumulates an exceptionally high level of free amino acids, especially lysine (Lys), threonine (Thr), methionine, and iso-leucine. In a cross between Oh545o2 and Oh51Ao2, we identified several quantitative trait loci linked with this phenotype. One of these is on the long arm of chromosome 2 and is linked with loci encoding aspartate (Asp) kinase 2 and Asp kinase (AK)-homoserine dehydrogenase (HSDH) 2. To investigate whether these enzymes can contribute to the high levels of Asp family amino acids, we measured their specific activity and feedback inhibition properties, as well as activities of several other key enzymes involved in Lys metabolism. We did not find a significant difference in total activity of dihydrodipicolinate synthase, HSDH, and Lys ketoglutarate reductase between these inbreds, and the feedback inhibition properties of HSDH and dihyrodipicolinate synthase by Lys and/or Thr were similar. The most significant difference we found between Oh545o2 and Oh51Ao2 is feedback inhibition of AK by Lys but not Thr. AK activity in Oh545o2 is less sensitive to Lys inhibition than that in Oh51Ao2, with a Lys I50 twice that of Oh51Ao2. AK activity in Oh545o2 endosperm is also higher than in Oh51Ao2 at 15 d after pollination, but not 20 d after pollination. The results indicate that the Lys-sensitive Asp kinase 2, rather than the Thr-sensitive AK-HSDH2, is the best candidate gene for the quantitative trait locus affecting free amino acid content in Oh545o2.
玉米（Zea mays）自交系Oh545o2积累了异常高水平的游离氨基酸，尤其是赖氨酸（Lys）、苏氨酸（Thr）、蛋氨酸和异亮氨酸。在Oh545o2和Oh51Ao2的杂交中，我们鉴定出了几个与该表型相关的数量性状基因座。其中一个位于2号染色体长臂上，与编码天冬氨酸（Asp）激酶2和天冬氨酸激酶（AK）-高丝氨酸脱氢酶（HSDH）2的基因座相连。为了研究这些酶是否能导致高水平的天冬氨酸家族氨基酸，我们测量了它们的比活性和反馈抑制特性，以及赖氨酸代谢中其他几种关键酶的活性。我们发现这些自交系之间二氢吡啶二羧酸合酶、HSDH和赖氨酸酮戊二酸还原酶的总活性没有显著差异，并且HSDH和二氢吡啶二羧酸合酶对赖氨酸和/或苏氨酸的反馈抑制特性相似。我们在Oh545o2和Oh51Ao2之间发现的最显著差异是赖氨酸对AK的反馈抑制，而苏氨酸则没有。Oh545o2中AK活性对赖氨酸抑制的敏感性低于Oh51Ao2，其赖氨酸I50是Oh51Ao2的两倍。授粉后15天，Oh545o2胚乳中的AK活性也高于Oh51Ao2，但授粉后20天则不然。结果表明，对赖氨酸敏感的天冬氨酸激酶2，而非对苏氨酸敏感的AK-HSDH2，是影响Oh545o2中游离氨基酸含量的数量性状基因座的最佳候选基因。
