Cardona Tanai
Department of Life Sciences, Imperial College London, London, UK.
Heliyon. 2018 Mar 6;4(3):e00548. doi: 10.1016/j.heliyon.2018.e00548. eCollection 2018 Mar.
When and how oxygenic photosynthesis originated remains controversial. Wide uncertainties exist for the earliest detection of biogenic oxygen in the geochemical record or the origin of water oxidation in ancestral lineages of the phylum Cyanobacteria. A unique trait of oxygenic photosynthesis is that the process uses a Type I reaction centre with a heterodimeric core, also known as Photosystem I, made of two distinct but homologous subunits, PsaA and PsaB. In contrast, all other known Type I reaction centres in anoxygenic phototrophs have a homodimeric core. A compelling hypothesis for the evolution of a heterodimeric Type I reaction centre is that the gene duplication that allowed the divergence of PsaA and PsaB was an adaptation to incorporate photoprotective mechanisms against the formation of reactive oxygen species, therefore occurring after the origin of water oxidation to oxygen. Here I show, using sequence comparisons and Bayesian relaxed molecular clocks that this gene duplication event may have occurred in the early Archean more than 3.4 billion years ago, long before the most recent common ancestor of crown group Cyanobacteria and the Great Oxidation Event. If the origin of water oxidation predated this gene duplication event, then that would place primordial forms of oxygenic photosynthesis at a very early stage in the evolutionary history of life.
氧光合作用何时以及如何起源仍存在争议。在地球化学记录中最早检测到生物源氧气或蓝细菌门祖先谱系中水分子氧化的起源方面，存在很大的不确定性。氧光合作用的一个独特特征是，该过程使用具有异源二聚体核心的I型反应中心，也称为光系统I，由两个不同但同源的亚基PsaA和PsaB组成。相比之下，所有其他已知的无氧光合生物中的I型反应中心都具有同型二聚体核心。关于异源二聚体I型反应中心进化的一个有说服力的假说是，使PsaA和PsaB发生分化的基因复制是一种适应机制，用于纳入针对活性氧形成的光保护机制，因此发生在水分子氧化为氧气之后。在这里，我通过序列比较和贝叶斯松弛分子钟表明，这一基因复制事件可能发生在34亿多年前的太古宙早期，远早于蓝细菌冠群的最近共同祖先和大氧化事件。如果水分子氧化的起源早于这一基因复制事件，那么氧光合作用的原始形式将处于生命进化历史的非常早期阶段。
