Khan Hameed, Archibald John M
Canadian Institute for Advanced Research, Program in Integrated Microbial Biodiversity, Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia, Canada.
Nucleic Acids Res. 2008 May;36(9):3043-53. doi: 10.1093/nar/gkn095. Epub 2008 Apr 8.
Cryptophytes are unicellular eukaryotic algae that acquired photosynthesis secondarily through the uptake and retention of a red-algal endosymbiont. The plastid genome of the cryptophyte Rhodomonas salina CCMP1319 was recently sequenced and found to contain a genetic element similar to a group II intron. Here, we explore the distribution, structure and function of group II introns in the plastid genomes of distantly and closely related cryptophytes. The predicted secondary structures of six introns contained in three different genes were examined and found to be generally similar to group II introns but unusually large in size (including the largest known noncoding intron). Phylogenetic analysis suggests that the cryptophyte group II introns were acquired via lateral gene transfer (LGT) from a euglenid-like species. Unexpectedly, the six introns occupy five distinct genomic locations, suggesting multiple LGT events or recent transposition (or both). Combined with structural considerations, RT-PCR experiments suggest that the transferred introns are degenerate 'twintrons' (i.e. nested group II/group III introns) in which the internal intron has lost its splicing capability, resulting in an amalgamation with the outer intron.
隐藻是单细胞真核藻类,它们通过摄取并保留红藻内共生体而次生获得光合作用。隐藻盐生红胞藻CCMP1319的质体基因组最近已被测序,发现其中含有一种类似于II类内含子的遗传元件。在这里,我们探究了远缘和近缘隐藻质体基因组中II类内含子的分布、结构和功能。对三个不同基因中所含的六个内含子的预测二级结构进行了检查,发现它们总体上与II类内含子相似,但尺寸异常大(包括已知最大的非编码内含子)。系统发育分析表明,隐藻的II类内含子是通过横向基因转移(LGT)从一种类似眼虫的物种获得的。出乎意料的是,这六个内含子占据了五个不同的基因组位置,表明发生了多次LGT事件或近期转座(或两者皆有)。结合结构方面的考虑,逆转录聚合酶链反应实验表明,转移的内含子是退化的“双内含子”(即嵌套的II类/III类内含子),其中内部内含子已失去其剪接能力,导致与外部内含子融合。