Cox M M, Soltis D A, Livneh Z, Lehman I R
J Biol Chem. 1983 Feb 25;258(4):2577-85.
RecA protein-promoted DNA strand exchange is greatly stimulated by the single-stranded DNA binding protein (SSB) of Escherichia coli. Stimulation is not a consequence of the binding of SSB to excess single-stranded DNA. It results instead from stabilization of recA protein-single-stranded DNA complexes formed in the presence of ATP and SSB. In the presence of SSB, recA protein does not measurably dissociate from these complexes for up to 90 min. However, in its absence, recA protein moves rapidly between two populations of single-stranded DNA, and complete equilibration occurs with t 1/2 of 17 s. Rapid transfer of recA protein to single-stranded DNA occurs during all stages of DNA strand exchange and does not require ATP. The transfer involves an equilibrium between free and bound recA protein rather than a direct redistribution between single-stranded DNA molecules. Thus, SSB prevents dissociation of recA protein from single-stranded DNA, rendering the binding of the recA protein to single-stranded DNA irreversible. Under these conditions, the pairing phase of the strand exchange reaction is accelerated to the point that it is no longer rate-limiting. These results can explain the relative inefficiency of DNA strand exchange in the absence of SSB.
大肠杆菌的单链DNA结合蛋白（SSB）能极大地促进RecA蛋白介导的DNA链交换。这种促进作用并非SSB与过量单链DNA结合的结果。相反，它是由在ATP和SSB存在下形成的RecA蛋白 - 单链DNA复合物的稳定化导致的。在有SSB存在的情况下，RecA蛋白在长达90分钟的时间内都不会明显从这些复合物中解离。然而，在没有SSB的情况下，RecA蛋白在单链DNA的两个群体之间快速移动，17秒的半衰期就能实现完全平衡。在DNA链交换的所有阶段，RecA蛋白都会快速转移到单链DNA上，且这一过程不需要ATP。这种转移涉及游离RecA蛋白和结合态RecA蛋白之间的平衡，而非单链DNA分子之间的直接重新分布。因此，SSB可防止RecA蛋白从单链DNA上解离，使RecA蛋白与单链DNA的结合变得不可逆。在这些条件下，链交换反应的配对阶段加速到不再是限速步骤。这些结果可以解释在没有SSB时DNA链交换相对低效的原因。
