Reinbothe S, Runge S, Reinbothe C, van Cleve B, Apel K
Department of Plant Genetics, Swiss Federal Institute of Technology Zurich (ETH).
Plant Cell. 1995 Feb;7(2):161-72. doi: 10.1105/tpc.7.2.161.
The key regulatory enzyme of chlorophyll biosynthesis in higher plants, the light-dependent NADPH:protochlorophyllide oxidoreductase (POR), is a nuclear-encoded plastid protein. Its post-translational transport into plastids is determined by its substrate. The precursor of POR (pPOR) is taken up and processed to mature size by plastids only in the presence of protochlorophyllide (Pchlide). In etioplasts, the endogenous level of Pchlide saturates the demands for pPOR translocation. During the light-induced transformation of etioplasts into chloroplasts, the Pchlide concentration declined drastically, and isolated chloroplasts rapidly lost the ability to import the precursor enzyme. The chloroplasts' import capacity for the pPOR, however, was restored when their intraplastidic level of Pchlide was raised by incubating the organelles in the dark with delta-aminolevulinic acid, a common precursor of tetrapyrroles. Additional evidence for the involvement of intraplastidic Pchlide in regulating the transport of pPOR into plastids was provided by experiments in which barley seedlings were grown under light/dark cycles. The intraplastidic Pchlide concentration in these plants underwent a diurnal fluctuation, with a minimum at the end of the day and a maximum at the end of the night period. Chloroplasts isolated at the end of the night translocated pPOR, whereas those isolated at the end of the day did not. Our results imply that the Pchlide-dependent transport of the pPOR into plastids might be part of a novel regulatory circuit by which greening plants fine tune both the enzyme and pigment levels, thereby avoiding the wasteful degradation of the imported pPOR as well as photodestruction of free Pchlide.
高等植物中叶绿素生物合成的关键调节酶，光依赖性NADPH：原叶绿素酸酯氧化还原酶（POR），是一种核编码的质体蛋白。其翻译后转运到质体中由其底物决定。只有在原叶绿素酸酯（Pchlide）存在的情况下，POR的前体（pPOR）才会被质体摄取并加工成成熟大小。在黄化质体中，Pchlide的内源水平满足了pPOR转运的需求。在黄化质体向叶绿体的光诱导转变过程中，Pchlide浓度急剧下降，分离的叶绿体迅速丧失了导入前体酶的能力。然而，当通过用δ-氨基乙酰丙酸（一种四吡咯的常见前体）在黑暗中孵育细胞器来提高其质体内的Pchlide水平时，叶绿体对pPOR的导入能力得以恢复。用大麦幼苗在光/暗周期下生长的实验提供了更多证据，证明质体内的Pchlide参与调节pPOR向质体的转运。这些植物中质体内的Pchlide浓度呈现昼夜波动，在白天结束时最低，在夜间结束时最高。在夜间结束时分离的叶绿体能够转运pPOR，而在白天结束时分离的叶绿体则不能。我们的结果表明，pPOR向质体的Pchlide依赖性转运可能是一种新型调节回路的一部分，通过该回路，绿化植物可以微调酶和色素水平，从而避免导入的pPOR的浪费性降解以及游离Pchlide的光破坏。
