Zeng Qinglu, Bonocora Richard P, Shub David A
Department of Biological Sciences, University at Albany, SUNY, Albany, NY 12222, USA.
Curr Biol. 2009 Feb 10;19(3):218-22. doi: 10.1016/j.cub.2008.11.069.
Homing endonuclease genes are mobile elements that promote their duplication into cognate sites that lack the endonuclease gene [1, 2]. The homing endonuclease initiates this event through site-specific DNA cleavage. Copying of the endonuclease gene follows as a consequence of DNA repair. A genome containing a homing endonuclease gene is subject to self-cleavage. Protection is accomplished through DNA sequence polymorphisms, as is the case in intronless homing of free-standing endonuclease genes [3, 4], or by disruption of the recognition site by a group I intron (or intein) into which the endonuclease ORF is embedded. We describe here a novel free-standing homing endonuclease from cyanobacteriophage S-PM2, which is similar to the DNA resolvase of bacteriophage T4 and is encoded adjacent to an intron-containing psbA gene [5, 6]. The endonuclease makes a specific double-strand cut near the intron insertion site (IIS), its DNA recognition site spans the IIS, and it is unable to cleave intron-containing psbA genes. This interdependence of a free-standing endonuclease gene and a group I intron, which we denote "collaborative homing," has not been reported previously and gives support to a hypothesis of formation of composite mobile introns by independent convergence of an intron and an endonuclease gene on the same target sequence.
归巢内切酶基因是可移动元件，可促进自身复制到缺乏内切酶基因的同源位点[1,2]。归巢内切酶通过位点特异性DNA切割引发这一过程。内切酶基因的复制是DNA修复的结果。含有归巢内切酶基因的基因组会发生自我切割。保护作用通过DNA序列多态性来实现，就像独立的内切酶基因在无内含子归巢中的情况一样[3,4]，或者通过I类内含子（或内含肽）破坏识别位点来实现，内切酶开放阅读框嵌入其中。我们在此描述了一种来自蓝藻噬菌体S-PM2的新型独立归巢内切酶，它类似于噬菌体T4的DNA解旋酶，且编码于一个含内含子的psbA基因附近[5,6]。该内切酶在内含子插入位点（IIS）附近进行特异性双链切割，其DNA识别位点跨越IIS，并且无法切割含内含子的psbA基因。我们将这种独立的内切酶基因与I类内含子之间的相互依存关系称为“协同归巢”，此前尚未有相关报道，这支持了一种关于内含子和内切酶基因在同一靶序列上通过独立汇聚形成复合可移动内含子的假说。
