Colleaux L, Michel-Wolwertz M R, Matagne R F, Dujon B
Département de Biologie Moléculaire, Institut Pasteur, Paris, France.
Mol Gen Genet. 1990 Sep;223(2):288-96. doi: 10.1007/BF00265065.
The mitochondrial DNA of the two interfertile algal species Chlamydomonas smithii and Chlamydomonas reinhardtii are co-linear with the exception of ca. 1 kb insertion (the alpha insert) present in C. smithii DNA only. In vegetative diploids resulting from interspecific crosses, mitochondrial genomes are transmitted biparentally except for the alpha insert which is transmitted to all C. reinhardtii molecules in a manner reminiscent of the intron-mediated conversion event that occurs at the omega locus in yeast mitochondria, under the action of the I-SceI endonuclease. Here we report that the alpha insert corresponds to a typical group I intron of 1075 bp, inserted within the gene for apocytochrome b and containing a 237 codon open reading frame (ORF). We also report the complete sequence of the apocytochrome b gene of C. smithii. Comparison with the sequence of the same gene in C. reinhardtii reveals the precise intron insertion site. These data, together with the previous genetic data provide the first example of intron mobility in mitochondria of the plant kingdom. The product of the intronic ORF shows 36% amino acid identity with the I-SceI endonuclease whereas the intron ribozyme shows a 60% identity at the nucleotide level with the Neurospora crassa cob.1 intron. The possibility of a recent horizontal transfer of introns between fungi and algae is discussed.
两种可杂交的藻类物种——史密斯衣藻（Chlamydomonas smithii）和莱茵衣藻（Chlamydomonas reinhardtii）的线粒体DNA是共线性的，只是史密斯衣藻的DNA中存在一个约1 kb的插入片段（α插入片段）。在种间杂交产生的营养二倍体中，线粒体基因组是双亲遗传的，但α插入片段除外，它以一种类似于酵母线粒体中ω位点发生的内含子介导的转换事件的方式，在I-SceI核酸内切酶的作用下，传递给所有莱茵衣藻的线粒体分子。在此我们报告，α插入片段对应于一个典型的1075 bp的I类内含子，插入到脱辅基细胞色素b基因内，包含一个237个密码子的开放阅读框（ORF）。我们还报告了史密斯衣藻脱辅基细胞色素b基因的完整序列。与莱茵衣藻中同一基因的序列比较揭示了精确的内含子插入位点。这些数据与先前的遗传数据一起，提供了植物王国线粒体中内含子移动性的首个例子。内含子ORF的产物与I-SceI核酸内切酶显示出36%的氨基酸同一性，而内含子核酶在核苷酸水平上与粗糙脉孢菌（Neurospora crassa）cob.1内含子显示出60%的同一性。文中讨论了真菌和藻类之间近期内含子水平转移的可能性。
