PDF(Pubmed)
Abstract：
Terrestrial ecosystems and human societies depend on oxygenic photosynthesis, which began to reshape our atmosphere approximately 2.5 billion years ago. The earliest known organisms carrying out oxygenic photosynthesis are the cyanobacteria, which use large complexes of phycobiliproteins as light-harvesting antennae. Phycobiliproteins rely on phycocyanobilin (PCB), a linear tetrapyrrole (bilin) chromophore, as the light-harvesting pigment that transfers absorbed light energy from phycobilisomes to the chlorophyll-based photosynthetic apparatus. Cyanobacteria synthesize PCB from heme in two steps: A heme oxygenase converts heme into biliverdin IXα (BV), and the ferredoxin-dependent bilin reductase (FDBR) PcyA then converts BV into PCB. In the current work, we examine the origins of this pathway. We demonstrate that PcyA evolved from pre-PcyA proteins found in nonphotosynthetic bacteria and that pre-PcyA enzymes are active FDBRs that do not yield PCB. Pre-PcyA genes are associated with two gene clusters. Both clusters encode bilin-binding globin proteins, phycobiliprotein paralogs that we designate as BBAGs (bilin biosynthesis-associated globins). Some cyanobacteria also contain one such gene cluster, including a BBAG, two V4R proteins, and an iron-sulfur protein. Phylogenetic analysis shows that this cluster is descended from those associated with pre-PcyA proteins and that light-harvesting phycobiliproteins are also descended from BBAGs found in other bacteria. We propose that PcyA and phycobiliproteins originated in heterotrophic, nonphotosynthetic bacteria and were subsequently acquired by cyanobacteria.
摘要：
陆地生态系统和人类社会依赖于氧气光合作用，大约在25亿年前开始重塑我们的大气层。已知最早的进行含氧光合作用的生物是蓝细菌，使用大型藻胆蛋白复合物作为光捕获天线。藻胆蛋白依赖于藻蓝蛋白（PCB），线性四吡咯（bilin）发色团，作为光收集色素，将吸收的光能量从藻胆体转移到基于叶绿素的光合装置。蓝细菌通过两个步骤从血红素合成PCB：血红素加氧酶将血红素转化为胆绿素IXα（BV），然后，铁氧还蛋白依赖性胆色素还原酶（FDBR）PcyA将BV转化为PCB。在目前的工作中,我们研究了这个途径的起源。我们证明了PcyA是从非光合细菌中发现的pre-PcyA蛋白进化而来的，并且pre-PcyA酶是不产生PCB的活性FDBR。Pre-PcyA基因与两个基因簇相关。两个簇都编码胆汁结合珠蛋白，我们指定为BBAG（bilin生物合成相关球蛋白）的藻胆蛋白旁系同源物。一些蓝藻还含有一个这样的基因簇，包括BBAG,两种V4R蛋白，和铁硫蛋白.系统发育分析表明，该簇是与PcyA前蛋白相关的簇的后代，并且光收获藻胆蛋白也是其他细菌中发现的BBAG的后代。我们认为PcyA和藻胆蛋白起源于异养，非光合细菌，随后被蓝细菌获得。
