Gray M W, Boer P H
Department of Biochemistry, Dalhousie University, Halifax, Nova Scotia, Canada.
Philos Trans R Soc Lond B Biol Sci. 1988 May 31;319(1193):135-47. doi: 10.1098/rstb.1988.0038.
The mitochondrial genome of Chlamydomonas reinhardtii, a unicellular green alga, is a linear 15.8 kilobase pair (kbp) molecule. In gene arrangement and mode of expression, as well as in size, it differs radically from the large (200-2400 kbp) mitochondrial genomes of higher plants. Heterologous hybridization experiments and nucleotide sequence analysis have revealed that C. reinhardtii mitochondrial DNA (mtDNA) is a compactly organized genome specifying at least eight proteins, a minimum of three transfer RNAs, and large subunit (LS) and small subunit (SS) ribosomal RNAs. Both strands of the mtDNA encode genetic information, with genes organized into perhaps a single transcriptional unit on each strand. Stable transcripts have been identified by Northern hybridization analysis, and transcript termini have been mapped by primer extension and S1 nuclease protection experiments. The results suggest that mature RNAs, which virtually saturate the genome, are generated by precise endonucleolytic cleavage of long precursors, with specific motifs (both primary sequence and secondary structure) implicated as processing signals. Codon usage in C. reinhardtii mitochondria is highly biased, with eight codons entirely absent from all protein-coding genes; however, even though codon usage is restricted, it appears that C. reinhardtii mtDNA cannot encode the minimum number of tRNAs needed to support mitochondrial protein synthesis. The most striking feature of C. reinhardtii mtDNA is the division of SS and LS rRNA genes into a number of separate subgenic coding segments ('modules') that are interspersed with one another and with protein-coding and tRNA genes. We have identified abundant small RNAs, transcribed from these modules, that approximate to the latter in size. This indicates that splicing of rRNA 'pieces' does not occur in this system. Rather, the mature rRNAs apparently exist and function as non-covalent complexes of small RNAs (four in SS rRNA, at least eight in LS rRNA), held together by intermolecular base pairing. These complexes contain all the conserved elements of the minimal secondary structures that define the functional core of conventional LS and SS rRNAs.
莱茵衣藻是一种单细胞绿藻，其线粒体基因组是一个由15.8千碱基对（kbp）组成的线性分子。在基因排列、表达模式以及大小方面，它与高等植物的大型（200 - 2400 kbp）线粒体基因组有根本差异。异源杂交实验和核苷酸序列分析表明，莱茵衣藻线粒体DNA（mtDNA）是一个紧密组织的基因组，至少指定了八种蛋白质、至少三种转运RNA以及大亚基（LS）和小亚基（SS）核糖体RNA。mtDNA的两条链都编码遗传信息，每条链上的基因可能组织成一个单一的转录单元。通过Northern杂交分析鉴定出了稳定的转录本，并通过引物延伸和S1核酸酶保护实验绘制了转录本末端图谱。结果表明，几乎充满基因组的成熟RNA是由长前体的精确内切核酸酶切割产生的，特定基序（包括一级序列和二级结构）被认为是加工信号。莱茵衣藻线粒体中的密码子使用存在高度偏向性，所有蛋白质编码基因中完全不存在八个密码子；然而，尽管密码子使用受到限制，但莱茵衣藻mtDNA似乎无法编码支持线粒体蛋白质合成所需的最少数量的tRNA。莱茵衣藻mtDNA最显著的特征是SS和LS rRNA基因被分成许多单独的亚基因编码片段（“模块”），这些片段相互穿插，并与蛋白质编码基因和tRNA基因相间排列。我们已经鉴定出从这些模块转录而来的大量小RNA，其大小与后者相近。这表明在这个系统中不存在rRNA“片段”的剪接情况。相反，成熟的rRNA显然以小RNA的非共价复合物形式存在并发挥功能（SS rRNA中有四个，LS rRNA中至少有八个），通过分子间碱基配对结合在一起。这些复合物包含定义传统LS和SS rRNA功能核心的最小二级结构的所有保守元件。
