Xiao H, Naktinis V, O'Donnell M
Microbiology Department, Cornell University Medical College, New York, New York 10021, USA.
J Biol Chem. 1995 Jun 2;270(22):13378-83. doi: 10.1074/jbc.270.22.13378.
The gamma complex (gamma delta delta' chi psi) and tau complex (tau delta delta' chi psi) clamp loaders require ATP hydrolysis to load beta sliding clamps onto DNA. The beta sliding clamp tethers the polymerase (Pol) III* replicase to DNA for processive synthesis. Pol III* contains both gamma and tau, but only one each of the delta, delta', chi, and psi subunits. Hence, there is ambiguity with respect to which clamp loader, the gamma or tau complex, exists in the Pol III* replicase structure. In this study, ATP-binding site mutants of gamma and tau have been prepared, and these mutants, when assembled into either the gamma or tau complex, are inactive in clamp loading. These mutants have been used as a tool to determine the identity of the clamp loader in Pol III*. The nine-subunit Pol III* has been assembled using either mutant gamma or tau in place of wild-type gamma or tau. The results show that mutation of gamma inactivates Pol III* activity, but mutation of tau does not, indicating that the gamma complex (and not the tau complex) is the clamp loader of Pol III*. The tau subunit carries the task of dimerizing the core polymerase, and it is this association of tau with core that appears to direct the single copy subunits away from tau and onto gamma.
γ复合体（γδδ'χψ）和τ复合体（τδδ'χψ）钳位装载器需要ATP水解才能将β滑动夹装载到DNA上。β滑动夹将聚合酶（Pol）III复制酶连接到DNA上以进行持续合成。Pol III同时包含γ和τ，但δ、δ'、χ和ψ亚基各只有一个。因此，关于Pol III复制酶结构中存在哪种钳位装载器，即γ复合体还是τ复合体，存在不确定性。在本研究中，制备了γ和τ的ATP结合位点突变体，这些突变体在组装成γ复合体或τ复合体时，在钳位装载方面无活性。这些突变体已被用作确定Pol III中钳位装载器身份的工具。已使用突变型γ或τ替代野生型γ或τ组装了九亚基的Pol III*。结果表明，γ的突变使Pol III活性失活，但τ的突变则不会，这表明γ复合体（而非τ复合体）是Pol III的钳位装载器。τ亚基承担着使核心聚合酶二聚化的任务，正是τ与核心的这种关联似乎将单拷贝亚基从τ引导到γ上。
