Denovan-Wright E M, Nedelcu A M, Lee R W
Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada.
Plant Mol Biol. 1998 Jan;36(2):285-95. doi: 10.1023/a:1005995718091.
The complete nucleotide sequence of the Chlamydomonas eugametos (Chlamydomonadales, Chlorophyceae, sensu Mattox and Stewart) mitochondrial genome has been determined (22,897 bp, 34.6% G + C). The genes identified in this circular-mapping genome include those for apocytochrome b, subunit 1 of the cytochrome oxidase complex, subunits 1, 2, 4, 5, and 6 of the NADH dehydrogenase complex, discontinuous large and small subunit ribosomal rRNAs and three tRNAs whose anticodons CAU, CCA and UUG are specific for methionine, tryptophan and glutamine, respectively. The C. eugametos mitochondrial DNA (mtDNA), therefore, shares almost the same reduced set of coding functions and similar unusual features of rRNA gene organization with the linear 15.8 kb mtDNA of Chlamydomonas reinhardtii, the only other completely sequenced chlamydomonadalean mtDNA. However, sequence analysis of the C. eugametos mtDNA has revealed the following distinguishing features relative to those of C. reinhardtii: (1) the absence of a reverse transcriptase-like gene homologue, (2) the presence of an additional gene for tRNA(met) that may be a pseudogene, (3) a completely different gene order, (4) transcription of all genes from the same mtDNA strand, (5) a lower G + C content, (6) less pronounced bias in codon usage, and (7) nine group I introns, several of which contain open reading frames coding for potential maturases/endonucleases and two have a nucleotide at the 5' or 3' splice site of the deduced precursor RNAs that deviates from highly conserved nucleotides reported in other group I introns. The features of mitochondrial genome organization and gene content shared by C. eugametos and C. reinhardtii contrast with those of other green algal mtDNAs that have been characterized in detail. The deep evolutionary divergence between these two Chlamydomonas taxa within the Chlamydomonadales suggests that their shared features of mitochondrial genome organization evolved prior to the origin of this group.
莱茵衣藻（衣藻目、绿藻纲，依据马托克斯和斯图尔特的分类法）线粒体基因组的完整核苷酸序列已被测定（22,897 bp，G + C含量为34.6%）。在这个呈环状图谱的基因组中鉴定出的基因包括脱辅基细胞色素b基因、细胞色素氧化酶复合体亚基1基因、NADH脱氢酶复合体亚基1、2、4、5和6基因、不连续的大、小亚基核糖体rRNA基因以及三个tRNA基因，其反密码子CAU、CCA和UUG分别对应甲硫氨酸、色氨酸和谷氨酰胺。因此，莱茵衣藻的线粒体DNA（mtDNA）与莱茵衣藻唯一另一个已完全测序的衣藻目线粒体DNA（15.8 kb线性mtDNA）几乎具有相同的精简编码功能集和相似的rRNA基因组织异常特征。然而，莱茵衣藻mtDNA的序列分析揭示了相对于莱茵衣藻mtDNA的以下显著特征：（1）不存在逆转录酶样基因同源物；（2）存在一个额外的tRNA(met)基因，可能是一个假基因；（3）基因顺序完全不同；（4）所有基因都从同一条mtDNA链转录；（5）G + C含量较低；（6）密码子使用偏好性不那么明显；（7）有九个I类内含子，其中几个含有编码潜在成熟酶/内切核酸酶的开放阅读框，并且有两个在推导的前体RNA的5'或3'剪接位点处有一个核苷酸，与其他I类内含子中报道的高度保守核苷酸不同。莱茵衣藻和莱茵衣藻共有的线粒体基因组组织和基因内容特征与其他已详细表征的绿藻mtDNA形成对比。衣藻目中这两个莱茵衣藻分类单元之间的深度进化分歧表明，它们线粒体基因组组织的共同特征在该类群起源之前就已演化形成。
