Turmel M, Boulanger J, Schnare M N, Gray M W, Lemieux C
Département de biochimie, Faculté des sciences et de génie, Université Laval, Québec, Canada.
J Mol Biol. 1991 Mar 20;218(2):293-311. doi: 10.1016/0022-2836(91)90713-g.
The chloroplast large subunit rRNA gene of Chlamydomonas eugametos and its 5' flanking region encoding tRNA(Ile) (GAU) and tRNA(Ala) (UGC) have been sequenced. The DNA sequence data along with the results of a detailed RNA analysis disclosed two unusual features of this green algal large subunit rRNA gene: (1) the presence of six group I introns (CeLSU.1-CeLSU.6) whose insertion positions have not been described previously, and (2) the presence of three short internal transcribed spacers that are post-transcriptionally excised to yield four rRNA species of 280, 52, 810 and 1720 nucleotides, positioned in this order (5' to 3') in the primary transcript. Together, these RNA species can assume a secondary structure that is almost identical to that proposed for the 23 S rRNA of Escherichia coli. All three internal transcribed spacers map to variable regions of primary sequence and/or potential secondary structure, whereas all six introns lie within highly conserved regions. The first three introns are inserted within the sequence encoding the 810 nucleotide rRNA species and map within domain II of the large subunit rRNA structure; the remaining introns, found in the sequence encoding the 1720 nucleotide rRNA species, lie within either domain IV or V, as is the case for all other large subunit rDNA introns that have been documented to date. CeLSU.5 and CeLSU.6 each contain a long open reading frame (ORF) of more than 200 codons. While the CeLSU.6 ORF is not related to any known ORFs, the CeLSU.5 ORF belongs to a family of ORFs that have been identified in Podospora and Neurospora mitochondrial group I introns. The finding that a polymorphic marker showing unidirectional gene conversion during crosses between C. eugametos and Chlamydomonas moewusii is located within the CeLSU.5 ORF makes it likely that this intron is a mobile element and that its ORF encodes a site-specific endonuclease promoting the transfer of the intron DNA sequence.
对衣藻（Chlamydomonas eugametos）叶绿体大亚基rRNA基因及其5'侧翼区域进行了测序，该区域编码tRNA（Ile）（GAU）和tRNA（Ala）（UGC）。DNA序列数据以及详细的RNA分析结果揭示了这种绿藻大亚基rRNA基因的两个不同寻常的特征：（1）存在六个I类内含子（CeLSU.1 - CeLSU.6），其插入位置此前未被描述；（2）存在三个短的内部转录间隔区，这些间隔区在转录后被切除，产生四种rRNA，分别为280、52、810和1720个核苷酸，在初级转录本中按此顺序（5'至3'）排列。这些RNA共同形成的二级结构几乎与大肠杆菌23S rRNA的二级结构相同。所有三个内部转录间隔区都位于一级序列和/或潜在二级结构的可变区域，而所有六个内含子都位于高度保守的区域。前三个内含子插入在编码810个核苷酸rRNA的序列中，并位于大亚基rRNA结构的结构域II内；其余的内含子存在于编码1720个核苷酸rRNA的序列中，位于结构域IV或V内，迄今为止记录的所有其他大亚基rDNA内含子也是如此。CeLSU.5和CeLSU.6各自包含一个超过200个密码子的长开放阅读框（ORF）。虽然CeLSU.6的ORF与任何已知的ORF都不相关，但CeLSU.5的ORF属于在Podospora和Neurospora线粒体I类内含子中鉴定出的一类ORF。在衣藻和莱茵衣藻（Chlamydomonas moewusii）杂交过程中显示单向基因转换的多态性标记位于CeLSU.5的ORF内，这一发现表明该内含子可能是一个可移动元件，其ORF编码一种位点特异性内切核酸酶，促进内含子DNA序列的转移。
