Stahl F, Myers R
Institute of Molecular Biology, University of Oregon, Eugene 97403-1229, USA.
J Hered. 1995 Sep-Oct;86(5):327-9. doi: 10.1093/oxfordjournals.jhered.a111599.
The DNA sequence 5'[GCTGGTGG]3', which is called chi, stimulates recombination that is mediated by the RecBCD pathway of Escherichia coli. In 1981, a model was proposed in which the RecBCD enzyme enters DNA at a double-chain end. The enzyme then travels between the chains by unwinding and rewinding the DNA at different rates so that the traveling enzyme becomes encumbered by a region of unwound DNA. Upon meeting chi, the enzyme was supposed to cut one of the two unwound chains, generating thereby a recombinagenic single-chain end. The model, based on microscopical observations of RecBCD enzyme interacting with linear duplex DNA, was supported by the subsequent finding that RecBCD acting in vitro under certain conditions did deliver a nick at chi. This widely embraced model has been challenged by a model in which the exonuclease activity of RecBCD destroys DNA from the enzyme's entry site to chi. The role of chi according to the new model is to inhibit this nuclease activity of RecBCD, perhaps by ejecting the RecD subunit from the enzyme, thereby revealing the enzyme's recombinase activity.
被称为χ的DNA序列5'[GCTGGTGG]3'可刺激由大肠杆菌RecBCD途径介导的重组。1981年，有人提出了一个模型，其中RecBCD酶在双链末端进入DNA。然后该酶通过以不同速率解开和重新缠绕DNA在两条链之间移动，使得移动的酶被一段解旋的DNA区域所阻碍。遇到χ时，该酶应该切割两条解旋链中的一条，从而产生一个重组单链末端。该模型基于对RecBCD酶与线性双链DNA相互作用的显微镜观察，随后发现RecBCD在某些条件下在体外确实在χ处产生一个切口，这支持了该模型。这个被广泛接受的模型受到了另一个模型的挑战，在这个模型中，RecBCD的核酸外切酶活性从酶的进入位点到χ破坏DNA。根据新模型，χ的作用是抑制RecBCD的这种核酸酶活性，可能是通过从酶中排出RecD亚基，从而揭示酶的重组酶活性。
