BACKGROUND:In the Chlorophyta--the green algal phylum comprising the classes Prasinophyceae, Ulvophyceae, Trebouxiophyceae and Chlorophyceae--the chloroplast genome displays a highly variable architecture. While chlorophycean chloroplast DNAs (cpDNAs) deviate considerably from the ancestral pattern described for the prasinophyte Nephroselmis olivacea, the degree of remodelling sustained by the two ulvophyte cpDNAs completely sequenced to date is intermediate relative to those observed for chlorophycean and trebouxiophyte cpDNAs. Chlorella vulgaris (Chlorellales) is currently the only photosynthetic trebouxiophyte whose complete cpDNA sequence has been reported. To gain insights into the evolutionary trends of the chloroplast genome in the Trebouxiophyceae, we sequenced cpDNA from the filamentous alga Leptosira terrestris (Ctenocladales). RESULTS:The 195,081-bp Leptosira chloroplast genome resembles the 150,613-bp Chlorella genome in lacking a large inverted repeat (IR) but differs greatly in gene order. Six of the conserved genes present in Chlorella cpDNA are missing from the Leptosira gene repertoire. The 106 conserved genes, four introns and 11 free standing open reading frames (ORFs) account for 48.3% of the genome sequence. This is the lowest gene density yet observed among chlorophyte cpDNAs. Contrary to the situation in Chlorella but similar to that in the chlorophycean Scenedesmus obliquus, the gene distribution is highly biased over the two DNA strands in Leptosira. Nine genes, compared to only three in Chlorella, have significantly expanded coding regions relative to their homologues in ancestral-type green algal cpDNAs. As observed in chlorophycean genomes, the rpoB gene is fragmented into two ORFs. Short repeats account for 5.1% of the Leptosira genome sequence and are present mainly in intergenic regions. CONCLUSION:Our results highlight the great plasticity of the chloroplast genome in the Trebouxiophyceae and indicate that the IR was lost on at least two separate occasions. The intriguing similarities of the derived features exhibited by Leptosira cpDNA and its chlorophycean counterparts suggest that the same evolutionary forces shaped the IR-lacking chloroplast genomes in these two algal lineages.

译文

背景:在绿藻(绿藻门,包括草藻科,绿藻科,绿藻科和绿藻科)中,叶绿体基因组显示出高度可变的结构。尽管叶绿藻叶绿体DNA(cpDNA)与描述的古藻类Nephroselmis olivacea的祖先模式有很大差异,但迄今为止,两个完全被测序的ulvophyte cpDNA所维持的重塑程度相对于叶绿藻和trebouxiophyte cpDNA所观察到的中等。小球藻(Chlorellales)是目前唯一已报道完整cpDNA序列的光合藻类植物。为了深入了解细叶藻科中叶绿体基因组的进化趋势,我们对丝状藻类海藻Leptosira terrestris(Ctenocladales)的cpDNA进行了测序。
结果:195081bp的叶绿体叶绿体基因组类似于150613bp的小球藻基因组,但缺少大的反向重复序列(IR),但基因顺序差异很大。小球藻cpDNA中存在的六个保守基因在Leptosira基因库中缺失。 106个保守基因,四个内含子和11个自由开放阅读框(ORF)占基因组序列的48.3%。这是迄今为止在绿藻cpDNA中观察到的最低基因密度。与小球藻中的情况相反,但与斜叶绿藻场景中的情况相似,该基因的分布在Leptosira的两条DNA链上高度偏向。与小球藻中的三个基因相比,九个基因相对于其祖先型绿色藻类cpDNA中的同源物具有显着扩展的编码区。如在叶绿素基因组中所观察到的,rpoB基因被片段化为两个ORF。短重复序列占Leptosira基因组序列的5.1%,主要存在于基因间区域。
结论:我们的结果突显了藻藻科中叶绿体基因组的可塑性,并表明至少在两次不同的情况下IR丢失了。 Leptosira cpDNA及其对应的叶绿素对应物表现出的令人着迷的相似之处表明,相同的进化力塑造了这两个藻类谱系中缺少IR的叶绿体基因组。

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