Bonocora Richard P, Shub David A
Department of Biological Sciences, State University of New York at Albany, Albany, NY 12222, USA.
Curr Biol. 2009 Feb 10;19(3):223-8. doi: 10.1016/j.cub.2009.01.033.
Mobile group I introns are RNA splicing elements that have been invaded by endonuclease genes. These endonucleases facilitate intron mobility by a unidirectional, duplicative gene-conversion process known as homing [1]. Survival of the invading endonuclease depends upon its ability to promote intron mobility. Therefore, the endonuclease must either quickly change its cleavage specificity to match the site of intron insertion, or it must already be preadapted to cleave this sequence. Here we show that the group I intron in the DNA polymerase gene of T7-like bacteriophage PhiI is mobile, dependent upon its intronic HNH homing endonuclease gene, I-TslI. We also show that gene 5.3 of phage T3, located adjacent to its intronless DNA polymerase gene, is a homologous homing endonuclease gene whose protein product initiates efficient spread of gene 5.3 into empty sites in related phages. Both of these endonucleases cleave intronless DNA polymerase genes at identical positions. This shared feature between an intronic and free-standing endonuclease is unprecedented. Based on this evidence, we propose that introns and their homing endonucleases evolve separately to target the same highly conserved sequences, uniting afterwards to create a composite mobile element.
I 组内含子是一类RNA剪接元件，已被内切核酸酶基因入侵。这些内切核酸酶通过一种称为归巢的单向复制基因转换过程促进内含子的移动性[1]。入侵的内切核酸酶的存活取决于其促进内含子移动的能力。因此，内切核酸酶必须要么迅速改变其切割特异性以匹配内含子插入位点，要么它必须已经预先适应切割该序列。在这里，我们表明，T7样噬菌体PhiI的DNA聚合酶基因中的I组内含子是可移动的，依赖于其内含子HNH归巢内切核酸酶基因I-TslI。我们还表明，噬菌体T3的5.3基因位于其无内含子的DNA聚合酶基因附近，是一个同源归巢内切核酸酶基因，其蛋白质产物启动5.3基因向相关噬菌体空位点的有效传播。这两种内切核酸酶都在无内含子的DNA聚合酶基因的相同位置切割。内含子内切核酸酶和独立内切核酸酶之间的这种共同特征是前所未有的。基于这一证据，我们提出内含子及其归巢内切核酸酶分别进化以靶向相同的高度保守序列，随后结合形成一个复合移动元件。
