Department of Biochemistry and Molecular Biology, Canadian Institute for Advanced Research, Integrated Microbial Biodiversity Program, Dalhousie University, Halifax, Nova Scotia, Canada.
J Eukaryot Microbiol. 2010 Nov-Dec;57(6):453-9. doi: 10.1111/j.1550-7408.2010.00511.x. Epub 2010 Oct 12.
Chlorarachniophytes are enigmatic marine unicellular algae that acquired photosynthesis by secondary endosymbiosis. Chlorarachniophytes are unusual in that the nucleus of the engulfed algal cell (a green alga) persists in a miniaturized form, termed a nucleomorph. The nucleomorph genome of the model chlorarachniophyte, Bigelowiella natans CCMP621, is 373 kilobase pairs (kbp) in size, the smallest nuclear genome characterized to date. The B. natans nucleomorph genome is composed of three chromosomes, each with canonical eukaryotic telomeres and sub-telomeric ribosomal DNA (rDNA) operons transcribed away from the chromosome end. Here we present the complete rDNA operon and telomeric region from the nucleomorph genome of Lotharella oceanica CCMP622, a newly characterized chlorarachniophyte strain with a genome ∼610 kbp in size, significantly larger than all other known chlorarachniophytes. We show that the L. oceanica rDNA operon is in the opposite chromosomal orientation to that of B. natans. Furthermore, we determined the rDNA operon orientation of five additional chlorarachniophyte strains, the majority of which possess the same arrangement as L. oceanica, with the exception of Chlorarachnion reptans and those very closely related to B. natans. It is thus possible that the ancestral rDNA operon orientation of the chlorarachniophyte nucleomorph genome might have been the same as in the independently evolved, red algal-derived, nucleomorph genomes of cryptophytes. A U2 small nuclear RNA gene was found adjacent to the telomere in Gymnochlora stellata CCMP2057 and Chlorarachnion sp. CCMP2014. This feature may represent a useful evolutionary character for inferring the relationships among extant lineages.
绿藻质体是一种神秘的海洋单细胞藻类，通过二次内共生获得光合作用。绿藻质体不同寻常之处在于，被吞噬的藻类细胞（一种绿藻）的核保持在微型化的形式，称为核质体。模型绿藻质体，Bigelowiella natans CCMP621 的核质体基因组大小为 373 千碱基对 (kbp)，是迄今为止特征化的最小核基因组。B. natans 核质体基因组由三个染色体组成，每个染色体都具有经典的真核端粒和亚端粒核糖体 DNA (rDNA) 操纵子，这些操纵子从染色体末端转录。在这里，我们展示了 Lotharella oceanica CCMP622 核质体基因组中完整的 rDNA 操纵子和端粒区，L. oceanica 是一种新描述的绿藻质体菌株，其基因组大小约为 610 kbp，明显大于所有其他已知的绿藻质体。我们表明，L. oceanica 的 rDNA 操纵子在染色体方向上与 B. natans 相反。此外，我们确定了另外五个绿藻质体菌株的 rDNA 操纵子方向，其中大多数与 L. oceanica 的排列相同，除了 Chlorarachnion reptans 和与 B. natans 非常密切相关的菌株。因此，绿藻质体核质体基因组的祖先 rDNA 操纵子方向可能与独立进化的、红藻衍生的隐藻核质体基因组中的方向相同。在 Gymnochlora stellata CCMP2057 和 Chlorarachnion sp. CCMP2014 中，我们发现 U2 小核 RNA 基因位于端粒附近。这一特征可能代表了推断现存谱系之间关系的有用进化特征。
