Braedt G, Smith G R
Fred Hutchinson Cancer Research Center, Seattle, WA 98104.
Proc Natl Acad Sci U S A. 1989 Feb;86(3):871-5. doi: 10.1073/pnas.86.3.871.
RecBCD enzyme (exonuclease V) of Escherichia coli unwinds DNA, frequently forming asymmetric structures with two single-stranded tails of unequal length abutting a single-stranded loop at the junction with double-stranded DNA. Their lengths are consistent with the longer tail being one strand of the duplex and the loop plus the shorter tail being the other strand. The strand polarity of the unwinding was determined by labeling the 3' or 5' ends of duplex DNA with biotinylated nucleotides, reacting the DNA with RecBCD enzyme, and distinguishing the labeled ends, in the electron microscope, by their binding to streptavidin-gold complex. The shorter tail was formed from the DNA strand with its 3' terminus at the duplex end where RecBCD enzyme entered. We conclude that RecBCD enzyme unwinds DNA by forming a loop on the strand with a 3' end at the entry point. This result is concordant with a previously proposed model of recombination, which we discuss.
大肠杆菌的RecBCD酶（外切核酸酶V）能解开DNA，经常形成不对称结构，在与双链DNA的连接处有两条长度不等的单链尾巴邻接一个单链环。它们的长度与较长的尾巴是双链中的一条链，而环加较短的尾巴是另一条链一致。通过用生物素化核苷酸标记双链DNA的3'或5'末端，使DNA与RecBCD酶反应，并在电子显微镜下通过它们与链霉亲和素-金复合物的结合来区分标记末端，从而确定了解旋的链极性。较短的尾巴由其3'末端位于RecBCD酶进入的双链末端的DNA链形成。我们得出结论，RecBCD酶通过在进入点具有3'末端的链上形成一个环来解开DNA。这一结果与之前提出的重组模型一致，我们将对此进行讨论。
