Tyndall C, Meister J, Bickle T A
Department of Microbiology, Basel University, Switzerland.
J Mol Biol. 1994 Apr 1;237(3):266-74. doi: 10.1006/jmbi.1994.1230.
The prr locus was originally described as coding a ribonuclease that is activated after phage T4 infection to cut within the anticodon of a specific tRNA, inactivating protein synthesis and thus blocking phage development. Wild-type T4 phage has two genes coding the enzymes polynucleotide kinase and RNA ligase, whose only function seems to be to repair the damage done by the anticodon nuclease. As the only apparent function of the prr ribonuclease is to combat phage infection, it can be considered as an RNA-based restriction enzyme. In non-infected cells, the prr enzyme is kept inactive in a complex with three other proteins which were predicted on the basis of DNA homologies to be the subunits of a type IC DNA restriction and modification system. Unlike other type IC systems so far characterized, prr is chromosomally rather than plasmid coded. However, sequences upstream from prr also have homology with sequences from the plasmid R124 and the prophage P1. We have now investigated the prr system and shown that it is indeed a bona fide type IC system which we call EcoprrI, and which is active both in vivo and in vitro. The system is fully functional even in the absence of the anticodon nuclease and seems to be a typical type I enzyme. EcoprrI recognizes the sequence CCA(N7)RTGC. One peculiarity is that, with low efficiency, EcoprrI will recognize and methylate variants of its recognition sequence such as CCT(N7)ATGC, which is methylated in one strand of the DNA only.
prr基因座最初被描述为编码一种核糖核酸酶，该酶在噬菌体T4感染后被激活，可在特定tRNA的反密码子内进行切割，从而使蛋白质合成失活，进而阻断噬菌体的发育。野生型T4噬菌体有两个基因编码多核苷酸激酶和RNA连接酶，它们唯一的功能似乎是修复反密码子核酸酶造成的损伤。由于prr核糖核酸酶唯一明显的功能是对抗噬菌体感染，因此它可被视为一种基于RNA的限制酶。在未感染的细胞中，prr酶与其他三种蛋白质形成复合物而保持无活性，这三种蛋白质根据DNA同源性预测为I型C类DNA限制与修饰系统的亚基。与迄今为止所描述的其他I型C类系统不同，prr是由染色体而非质粒编码的。然而，prr上游的序列也与质粒R124和原噬菌体P1的序列具有同源性。我们现在对prr系统进行了研究，结果表明它确实是一个货真价实的I型C类系统，我们将其称为EcoprrI，它在体内和体外均具有活性。即使没有反密码子核酸酶，该系统仍能完全发挥功能，似乎是一种典型的I型酶。EcoprrI识别序列CCA(N7)RTGC。一个特殊之处在于，EcoprrI会以较低的效率识别并甲基化其识别序列的变体，如CCT(N7)ATGC，该序列仅在DNA的一条链上被甲基化。
