Cherry J R, Hondred D, Walker J M, Vierstra R D
Department of Horticulture, University of Wisconsin-Madison, WI 53706.
Proc Natl Acad Sci U S A. 1992 Jun 1;89(11):5039-43. doi: 10.1073/pnas.89.11.5039.
Phytochrome is a red/far-red-absorbing photoreceptor that controls many aspects of plant photomorphogenesis. Because proteolytic removal of approximately 6 kDa from the N terminus of 124-kDa oat phytochrome substantially alters many physicochemical properties of the chromoprotein, it has been proposed that the N terminus is required for biological activity. Here we test this hypothesis by comparing tobacco plants expressing full-length oat phytochrome (FL) with plants expressing a 118-kDa oat phytochrome lacking amino acids 7-69 (NA phytochrome). NA phytochrome, like its FL counterpart, exists as a homodimer in solution, is capable of covalently binding chromophore to form a red/far-red-photoreversible product, and is rapidly degraded in vivo after photoconversion to the far-red-absorbing form. However, like proteolytically degraded phytochrome missing the N terminus, the absorption maxima of the red- and far-red-light-absorbing forms of NA phytochrome are blue shifted relative to the maxima of the FL chromoprotein, and the rate of dark reversion of the far-red- to red-light-absorbing form is substantially increased. Tobacco plants producing high levels of NA phytochrome do not exhibit the light-exaggerated phenotype characteristic of FL phytochrome overexpression. By comparison of phytochrome-dose-phenotype-response curves generated by using a series of transgenic lines expressing various levels of FL or NA phytochrome, we demonstrate that NA phytochrome has less than 1/5th the biological activity of FL phytochrome expressed in tobacco. Furthermore, the shape of the dose-response curve for plants expressing FL phytochrome indicates that there is a sharp transition between phenotypically normal and abnormal plants over a relatively narrow range of phytochrome content, demonstrating that precise control of phytochrome levels is critical to photomorphogenesis.
光敏色素是一种吸收红光/远红光的光感受器，它控制着植物光形态建成的许多方面。由于从124 kDa燕麦光敏色素的N端蛋白水解去除约6 kDa会显著改变该色素蛋白的许多物理化学性质，因此有人提出N端对于生物活性是必需的。在这里，我们通过比较表达全长燕麦光敏色素（FL）的烟草植株与表达缺少氨基酸7 - 69的118 kDa燕麦光敏色素（NA光敏色素）的植株来检验这一假设。NA光敏色素与其FL对应物一样，在溶液中以同型二聚体形式存在，能够共价结合发色团形成红光/远红光光可逆产物，并且在光转化为远红光吸收形式后在体内迅速降解。然而，与蛋白水解降解缺失N端的光敏色素一样，NA光敏色素的红光和远红光吸收形式的吸收最大值相对于FL色素蛋白的最大值发生了蓝移，并且远红光到红光吸收形式的暗逆转速率大幅增加。产生高水平NA光敏色素的烟草植株不表现出FL光敏色素过表达所特有的光夸大表型。通过比较使用一系列表达不同水平FL或NA光敏色素的转基因系生成的光敏色素剂量-表型-反应曲线，我们证明NA光敏色素的生物活性不到烟草中表达的FL光敏色素的1/5。此外，表达FL光敏色素的植物的剂量反应曲线形状表明，在相对较窄的光敏色素含量范围内，表型正常和异常的植物之间存在急剧转变，这表明精确控制光敏色素水平对光形态建成至关重要。
