Bertucat G, Lavery R, Prévost C
Laboratoire de Biochimie Théorique, CNRS UPR 9080, Institut de Biologie Physico-Chimique, 75005 Paris, France.
Biophys J. 1999 Sep;77(3):1562-76. doi: 10.1016/S0006-3495(99)77004-9.
A number of studies have concluded that strand exchange between a RecA-complexed DNA single strand and a homologous DNA duplex occurs via a single-strand invasion of the minor groove of the duplex. Using molecular modeling, we have previously demonstrated the possibility of forming a parallel triple helix in which the single strand interacts with the intact duplex in the minor groove, via novel base interactions (Bertucat et al., J. Biomol. Struct. Dynam. 16:535-546). This triplex is stabilized by the stretching and unwinding imposed by RecA. In the present study, we show that the bases within this triplex are appropriately placed to undergo strand exchange. Strand exchange is found to be exothermic and to result in a triple helix in which the new single strand occupies the major groove. This structure, which can be equated to so-called R-form DNA, can be further stabilized by compression and rewinding. We are consequently able to propose a detailed, atomic-scale model of RecA-promoted strand exchange. This model, which is supported by a variety of experimental data, suggests that the role of RecA is principally to prepare the single strand for its future interactions, to guide a minor groove attack on duplex DNA, and to stabilize the resulting, stretched triplex, which intrinsically favors strand exchange. We also discuss how this mechanism can incorporate homologous recognition.
多项研究得出结论，RecA 复合的 DNA 单链与同源 DNA 双链体之间的链交换是通过双链体小沟的单链侵入发生的。我们之前利用分子建模证明了形成平行三链螺旋的可能性，其中单链通过新型碱基相互作用与小沟中的完整双链体相互作用（Bertucat 等人，《生物分子结构与动力学杂志》16:535 - 546）。这种三链螺旋通过 RecA 施加的拉伸和展开得以稳定。在本研究中，我们表明该三链螺旋内的碱基位置合适，可进行链交换。发现链交换是放热的，并导致形成一种三链螺旋，其中新的单链占据大沟。这种结构可等同于所谓的 R 型 DNA，可通过压缩和重新缠绕进一步稳定。因此，我们能够提出一个 RecA 促进链交换的详细原子尺度模型。该模型得到了各种实验数据的支持，表明 RecA 的作用主要是为单链未来的相互作用做准备，引导对双链 DNA 的小沟攻击，并稳定由此产生的拉伸三链螺旋，而这种三链螺旋本质上有利于链交换。我们还讨论了这种机制如何纳入同源识别。
