Haugen Peik, Wikmark Odd-Gunnar, Vader Anna, Coucheron Dag H, Sjøttem Eva, Johansen Steinar D
Department of Molecular Biotechnology, RNA Research Group, Institute of Medical Biology, University of Tromsø N-9037 Tromsø, Norway.
Nucleic Acids Res. 2005 May 12;33(8):2734-41. doi: 10.1093/nar/gki564. Print 2005.
The myxomycete Didymium iridis (isolate Panama 2) contains a mobile group I intron named Dir.S956-1 after position 956 in the nuclear small subunit (SSU) rRNA gene. The intron is efficiently spread through homing by the intron-encoded homing endonuclease I-DirI. Homing endonuclease genes (HEGs) usually spread with their associated introns as a unit, but infrequently also spread independent of introns (or inteins). Clear examples of HEG mobility are however sparse. Here, we provide evidence for the transfer of a HEG into a group I intron named Dir.S956-2 that is inserted into the SSU rDNA of the Costa Rica 8 isolate of D.iridis. Similarities between intron sequences that flank the HEG and rDNA sequences that flank the intron (the homing endonuclease recognition sequence) suggest that the HEG invaded the intron during the recent evolution in a homing-like event. Dir.S956-2 is inserted into the same SSU site as Dir.S956-1. Remarkably, the two group I introns encode distantly related splicing ribozymes with phylogenetically related HEGs inserted on the opposite strands of different peripheral loop regions. The HEGs are both interrupted by small spliceosomal introns that must be removed during RNA maturation.
黏菌双孢绒泡菌（巴拿马2分离株）含有一个可移动的I组内含子，根据其在核小亚基（SSU）rRNA基因中的位置956，命名为Dir.S956 - 1。该内含子通过内含子编码的归巢内切酶I - DirI高效地通过归巢进行传播。归巢内切酶基因（HEGs）通常与其相关的内含子作为一个单位传播，但偶尔也会独立于内含子（或内蛋白子）传播。然而，HEG移动性的明确例子很少。在这里，我们提供了证据，证明一个HEG转移到了一个名为Dir.S956 - 2的I组内含子中，该内含子插入到了双孢绒泡菌哥斯达黎加8分离株的SSU rDNA中。HEG侧翼的内含子序列与内含子侧翼的rDNA序列（归巢内切酶识别序列）之间的相似性表明，在最近的进化过程中，HEG以类似归巢的事件侵入了内含子。Dir.S956 - 2插入到与Dir.S956 - 1相同的SSU位点。值得注意的是，这两个I组内含子编码远缘相关的剪接核酶，在不同外周环区域的相反链上插入了系统发育相关的HEG。这两个HEG都被小核核糖核酸内含子打断，在RNA成熟过程中必须将其去除
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