Llorente Briardo, de Souza Flavio S J, Soto Gabriela, Meyer Cristian, Alonso Guillermo D, Flawiá Mirtha M, Bravo-Almonacid Fernando, Ayub Nicolás D, Rodríguez-Concepción Manuel
Centre for Research in Agricultural Genomics (CRAG) CSIC-IRTA-UAB-UB, 08193 Barcelona, Spain.
Instituto de Investigaciones en Ingeniería Genética y Biología Molecular Dr. Héctor Torres, Consejo Nacional de Investigaciones Científicas y Técnicas (INGEBI-CONICET), C1428ADN Buenos Aires, Argentina.
Sci Rep. 2016 Jan 11;6:19036. doi: 10.1038/srep19036.
The plastid organelle comprises a high proportion of nucleus-encoded proteins that were acquired from different prokaryotic donors via independent horizontal gene transfers following its primary endosymbiotic origin. What forces drove the targeting of these alien proteins to the plastid remains an unresolved evolutionary question. To better understand this process we screened for suitable candidate proteins to recapitulate their prokaryote-to-eukaryote transition. Here we identify the ancient horizontal transfer of a bacterial polyphenol oxidase (PPO) gene to the nuclear genome of an early land plant ancestor and infer the possible mechanism behind the plastidial localization of the encoded enzyme. Arabidopsis plants expressing PPO versions either lacking or harbouring a plastid-targeting signal allowed examining fitness consequences associated with its subcellular localization. Markedly, a deleterious effect on plant growth was highly correlated with PPO activity only when producing the non-targeted enzyme, suggesting that selection favoured the fixation of plastid-targeted protein versions. Our results reveal a possible evolutionary mechanism of how selection against heterologous genes encoding cytosolic proteins contributed in incrementing plastid proteome complexity from non-endosymbiotic gene sources, a process that may also impact mitochondrial evolution.
质体细胞器包含很大比例的由细胞核编码的蛋白质，这些蛋白质是在其最初的内共生起源之后，通过独立的水平基因转移从不同的原核供体获得的。是什么力量驱使这些外来蛋白质靶向质体，这仍然是一个未解决的进化问题。为了更好地理解这个过程，我们筛选了合适的候选蛋白质来重现它们从原核生物到真核生物的转变。在这里，我们确定了一种细菌多酚氧化酶（PPO）基因向早期陆地植物祖先的核基因组的古老水平转移，并推断了编码酶的质体定位背后的可能机制。表达缺乏或含有质体靶向信号的PPO版本的拟南芥植物，使得我们能够研究与其亚细胞定位相关的适应性后果。值得注意的是，只有在产生非靶向酶时，对植物生长的有害影响才与PPO活性高度相关，这表明选择有利于质体靶向蛋白版本的固定。我们的结果揭示了一种可能的进化机制，即针对编码胞质蛋白的异源基因的选择如何有助于增加来自非内共生基因源的质体蛋白质组复杂性，这一过程也可能影响线粒体进化。
