López de Saro Francisco J, Georgescu Roxana E, O'Donnell Mike
Howard Hughes Medical Institute and Laboratory of DNA Replication, The Rockefeller University, 1230 York Avenue, New York, NY 10021-6399, USA.
Proc Natl Acad Sci U S A. 2003 Dec 9;100(25):14689-94. doi: 10.1073/pnas.2435454100. Epub 2003 Nov 20.
Chromosomal DNA polymerases are tethered to DNA by a circular sliding clamp for high processivity. However, lagging strand synthesis requires the polymerase to rapidly dissociate on finishing each Okazaki fragment. The Escherichia coli replicase contains a subunit (tau) that promotes separation of polymerase from its clamp on finishing DNA segments. This report reveals the mechanism of this process. We find that tau binds the C-terminal residues of the DNA polymerase. Surprisingly, this same C-terminal "tail" of the polymerase interacts with the beta clamp, and tau competes with beta for this sequence. Moreover, tau acts as a DNA sensor. On binding primed DNA, tau releases the polymerase tail, allowing polymerase to bind beta for processive synthesis. But on sensing the DNA is complete (duplex), tau sequesters the polymerase tail from beta, disengaging polymerase from DNA. Therefore, DNA sensing by tau switches the polymerase peptide tail on and off the clamp and coordinates the dynamic turnover of polymerase during lagging strand synthesis.
染色体DNA聚合酶通过一个环状滑动夹与DNA相连，以实现高保真性。然而，滞后链的合成要求聚合酶在完成每个冈崎片段后迅速解离。大肠杆菌复制酶包含一个亚基（tau），它能促进聚合酶在完成DNA片段后与其夹子分离。本报告揭示了这一过程的机制。我们发现tau与DNA聚合酶的C末端残基结合。令人惊讶的是，聚合酶的这个相同的C末端“尾巴”与β夹子相互作用，并且tau与β竞争该序列。此外，tau作为一种DNA传感器。在结合引发的DNA时，tau释放聚合酶尾巴，使聚合酶能够结合β进行持续性合成。但在检测到DNA完成（双链）时，tau将聚合酶尾巴从β中隔离出来，使聚合酶与DNA分离。因此，tau对DNA的感应开启和关闭了聚合酶肽尾巴与夹子的结合，并在滞后链合成过程中协调聚合酶的动态周转。
