Rao B J, Dutreix M, Radding C M
Department of Human Genetics, Yale University School of Medicine, New Haven, CT 06510.
Proc Natl Acad Sci U S A. 1991 Apr 15;88(8):2984-8. doi: 10.1073/pnas.88.8.2984.
When RecA protein, in the form of a nucleoprotein filament containing circular single-stranded DNA (plus strand only), reacts with homologous linear duplex DNA, a directional transfer ensues of a strand from the duplex DNA to the nucleoprotein filament, resulting in the displacement of the linear plus strand in the 5' to 3' direction. The initial homologous synapsis, however, can occur at either end of the duplex DNA, or anywhere in between, and when homology is restricted to different regions of the duplex DNA, the joint molecules that form in each region show striking differences in stability upon deproteinization: distal joints greater than proximal joints much greater than medial joints. In the deproteinized distal joints, which are thermostable, 2000 nucleotide residues of the circular plus strand are resistant to P1 nuclease; both strands of the original duplex DNA remain resistant to P1 nuclease, and the potentially displaceable linear plus strand, which has a 3' homologous end, remains resistant to Escherichia coli exonuclease I. These observations suggest that RecA protein promotes homologous pairing and strand exchange via long three-stranded DNA intermediates and, moreover, that, once formed, such triplex structures in natural DNA are stable even when RecA protein has been removed.
当以包含环状单链DNA（仅正链）的核蛋白细丝形式存在的RecA蛋白与同源线性双链DNA反应时，双链DNA中的一条链会向核蛋白细丝进行定向转移，导致线性正链在5'至3'方向上被取代。然而，最初的同源联会可以发生在双链DNA的任何一端，或两端之间的任何位置，并且当同源性局限于双链DNA的不同区域时，在每个区域形成的连接分子在脱蛋白后表现出显著的稳定性差异：远端连接大于近端连接，近端连接远大于中间连接。在热稳定的脱蛋白远端连接中，环状正链的2000个核苷酸残基对P1核酸酶具有抗性；原始双链DNA的两条链对P1核酸酶仍具有抗性，并且具有3'同源末端的潜在可取代线性正链对大肠杆菌核酸外切酶I仍具有抗性。这些观察结果表明，RecA蛋白通过长的三链DNA中间体促进同源配对和链交换，而且，一旦形成，天然DNA中的这种三链结构即使在去除RecA蛋白后仍然稳定。
