Turmel Monique, Otis Christian, Lemieux Claude
Département de Biochimie et de Microbiologie, Canadian Institute for Advanced Research, Pavillon C.-E. Marchand, Université Laval, Québec G1K 7P4, Canada.
Mol Biol Evol. 2002 Jan;19(1):24-38. doi: 10.1093/oxfordjournals.molbev.a003979.
To gain insights into the nature of the mitochondrial genome in the common ancestor of all green plants, we have completely sequenced the mitochondrial DNA (mtDNA) of Mesostigma viride. This green alga belongs to a morphologically heterogeneous class (Prasinophyceae) that includes descendants of the earliest diverging green plants. Recent phylogenetic analyses of ribosomal RNAs (rRNAs) and concatenated proteins encoded by the chloroplast genome identified Mesostigma as a basal branch relative to the Streptophyta and the Chlorophyta, the two phyla that were previously thought to contain all extant green plants. The circular mitochondrial genome of Mesostigma resembles the mtDNAs of green algae occupying a basal position within the Chlorophyta in displaying a small size (42,424 bp) and a high gene density (86.6% coding sequences). It contains 65 genes that are conserved in other mtDNAs. Although none of these genes represents a novel coding sequence among green plant mtDNAs, four of them (rps1, sdh3, sdh4, and trnL[caa]) have not been reported previously in chlorophyte mtDNAs, and two others (rpl14 and trnI[gau]) have not been identified in the streptophyte mtDNAs examined so far (land-plant mtDNAs). Phylogenetic analyses of 19 concatenated mtDNA-encoded proteins favor the hypothesis that Mesostigma represents the earliest branch of green plant evolution. Four group I introns (two in rnl and two in cox1) and three group II introns (two in nad3 and one in cox2), two of which are trans-spliced at the RNA level, reside in Mesostigma mtDNA. The insertion sites of the three group II introns are unique to this mtDNA, suggesting that trans-splicing arose independently in the Mesostigma lineage and in the Streptophyta. The few structural features that can be regarded as ancestral in Mesostigma mtDNA predict that the common ancestor of all green plants had a compact mtDNA containing a minimum of 75 genes and perhaps two group I introns. Considering that the mitochondrial genome is much larger in size in land plants than in Mesostigma, we infer that mtDNA size began to increase dramatically in the Streptophyta either during the evolution of charophyte green algae or during the transition from charophytes to land plants.
为深入了解所有绿色植物共同祖先中线粒体基因组的本质，我们对中带藻（Mesostigma viride）的线粒体DNA（mtDNA）进行了全序列测定。这种绿藻属于形态各异的一类（原绿藻纲），包括最早分化的绿色植物的后代。最近对叶绿体基因组编码的核糖体RNA（rRNA）和串联蛋白进行的系统发育分析确定，相对于链形植物和绿藻门，中带藻是一个基部分支，此前认为这两个门包含了所有现存的绿色植物。中带藻的环状线粒体基因组类似于绿藻门中处于基部位置的绿藻的mtDNA，其特点是尺寸小（42,424 bp）且基因密度高（编码序列占86.6%）。它包含65个在其他mtDNA中保守的基因。尽管这些基因在绿色植物mtDNA中均不代表新的编码序列，但其中四个基因（rps1、sdh3、sdh4和trnL[caa]）此前在绿藻mtDNA中未被报道，另外两个基因（rpl14和trnI[gau]）在迄今为止所检测的链形植物mtDNA（陆地植物mtDNA）中尚未被鉴定出来。对19个串联的mtDNA编码蛋白进行的系统发育分析支持这样一种假说，即中带藻代表了绿色植物进化的最早分支。中带藻mtDNA中有四个I组内含子（两个在rnl中，两个在cox1中）和三个II组内含子（两个在nad3中，一个在cox2中），其中两个在RNA水平上进行反式剪接。这三个II组内含子的插入位点是该mtDNA所特有的，表明反式剪接在中带藻谱系和链形植物中是独立出现的。中带藻mtDNA中少数可被视为祖先特征的结构特征预示，所有绿色植物的共同祖先拥有一个紧凑的mtDNA，至少包含75个基因，可能还有两个I组内含子。鉴于陆地植物的线粒体基因组尺寸比中带藻大得多，我们推断，mtDNA尺寸在轮藻绿藻进化过程中或从轮藻向陆地植物转变过程中开始在链形植物中急剧增加。
