Ruf S, Kössel H, Bock R
Institut für Biologie III, Universität Freiburg, Germany.
J Cell Biol. 1997 Oct 6;139(1):95-102. doi: 10.1083/jcb.139.1.95.
The chloroplast genome of all higher plants encodes, in its large single-copy region, a conserved open reading frame of unknown function (ycf3), which is split by two group II introns and undergoes RNA editing in monocotyledonous plants. To elucidate the function of ycf3 we have deleted the reading frame from the tobacco plastid genome by biolistic transformation. We show here that homoplasmic Deltaycf3 plants display a photosynthetically incompetent phenotype. Molecular analyses indicate that this phenotype is not due to a defect in any of the general functions of the plastid genetic apparatus. Instead, the mutant plants specifically lack detectable amounts of all photosystem I (PSI) subunits analyzed. In contrast, at least under low light conditions, photosystem II subunits are still present and assemble into a physiologically active complex. Faithful transcription of photosystem I genes as well as correct mRNA processing and efficient transcript loading with ribosomes in the Deltaycf3 plants suggest a posttranslational cause of the PSI-defective phenotype. We therefore propose that ycf3 encodes an essential protein for the assembly and/or stability of functional PSI units. This study provides a first example for the suitability of reverse genetics approaches to complete our picture of the coding capacity of higher plant chloroplast genomes.
所有高等植物的叶绿体基因组在其大单拷贝区域编码一个功能未知的保守开放阅读框（ycf3），该阅读框被两个II类内含子分割，并且在单子叶植物中会发生RNA编辑。为了阐明ycf3的功能，我们通过生物枪转化从烟草质体基因组中删除了该阅读框。我们在此表明，同质的Δycf3植物表现出光合无能的表型。分子分析表明，这种表型不是由于质体遗传装置的任何一般功能缺陷所致。相反，突变植物特别缺乏所分析的所有光系统I（PSI）亚基的可检测量。相比之下，至少在弱光条件下，光系统II亚基仍然存在并组装成生理活性复合物。Δycf3植物中光系统I基因的忠实转录以及正确的mRNA加工和核糖体对转录本的有效加载表明PSI缺陷表型是翻译后原因导致的。因此，我们提出ycf3编码一种对功能性PSI单元的组装和/或稳定性至关重要的蛋白质。这项研究为反向遗传学方法适用于完善我们对高等植物叶绿体基因组编码能力的认识提供了首个实例。
