Suzuki J Y, Bauer C E
Department of Biology, Indiana University, Bloomington 47405, USA.
Proc Natl Acad Sci U S A. 1995 Apr 25;92(9):3749-53. doi: 10.1073/pnas.92.9.3749.
Flowering plants require light for chlorophyll synthesis. Early studies indicated that the dependence on light for greening stemmed in part from the light-dependent reduction of the chlorophyll intermediate protochlorophyllide to the product chlorophyllide. Light-dependent reduction of protochlorophyllide by flowering plants is contrasted by the ability of nonflowering plants, algae, and photosynthetic bacteria to reduce protochlorophyllide and, hence, synthesize (bacterio) chlorophyll in the dark. In this report, we functionally complemented a light-independent protochlorophyllide reductase mutant of the eubacterium Rhodobacter capsulatus with an expression library composed of genomic DNA from the cyanobacterium Synechocystis sp. PCC 6803. The complemented R. capsulatus strain is capable of synthesizing bacteriochlorophyll in the light, thereby indicating that a chlorophyll biosynthesis enzyme can function in the bacteriochlorophyll biosynthetic pathway. However, under dark growth conditions the complemented R. capsulatus strain fails to synthesize bacteriochlorophyll and instead accumulates protochlorophyllide. Sequence analysis demonstrates that the complementing Synechocystis genomic DNA fragment exhibits a high degree of sequence identity (53-56%) with light-dependent protochlorophyllide reductase enzymes found in plants. The observation that a plant-type, light-dependent protochlorophyllide reductase enzyme exists in a cyanobacterium indicates that light-dependent protochlorophyllide reductase evolved before the advent of eukaryotic photosynthesis. As such, this enzyme did not arise to fulfill a function necessitated either by the endosymbiotic evolution of the chloroplast or by multicellularity; rather, it evolved to fulfill a fundamentally cell-autonomous role.
开花植物需要光照来合成叶绿素。早期研究表明，对光照进行绿化的依赖部分源于叶绿素中间体原叶绿素酸酯向产物叶绿素酸酯的光依赖性还原。开花植物对原叶绿素酸酯的光依赖性还原与非开花植物、藻类和光合细菌在黑暗中还原原叶绿素酸酯并因此合成（细菌）叶绿素的能力形成对比。在本报告中，我们用由集胞藻属蓝细菌PCC 6803的基因组DNA组成的表达文库对真细菌红假单胞菌的一个光独立原叶绿素酸酯还原酶突变体进行了功能互补。互补后的红假单胞菌菌株能够在光照下合成细菌叶绿素，从而表明一种叶绿素生物合成酶可以在细菌叶绿素生物合成途径中发挥作用。然而，在黑暗生长条件下，互补后的红假单胞菌菌株无法合成细菌叶绿素，而是积累原叶绿素酸酯。序列分析表明，互补的集胞藻基因组DNA片段与植物中发现的光依赖性原叶绿素酸酯还原酶具有高度的序列同一性（53 - 56%）。在蓝细菌中存在植物型光依赖性原叶绿素酸酯还原酶这一观察结果表明，光依赖性原叶绿素酸酯还原酶在真核光合作用出现之前就已经进化。因此，这种酶的出现并非是为了履行叶绿体共生进化或多细胞性所必需的功能；相反，它的进化是为了履行一种基本的细胞自主功能。
