Blumenthal T, Young R A, Brown S
J Biol Chem. 1976 May 10;251(9):2740-3.
Qbeta replicase is a complex of four nonidentical subunits readily dissociable into two subcomplexes: 30 S ribosomal protein S1 and the phage-coded polypeptide (Subunits I + II) and protein synthesis elongation factors EF-Tu and EF-Ts (Subunits III + IV). The affinity of the two subcomplexes for one another increases with increasing ionic strength. The enzyme is capable of initiation of RNA synthesis with synthetic templates only when in the low ionic strength conformation. Elongation of initiated polynucleotide chains is not affectedby ionic strength. Addition of Qbeta RNA to the enzyme also alters its quaternary structure: the EF-Tu-Ts cannot be covalently attached to the other enzyme subunits with bifunctional cross-linking reagents in the presence of RNA. This conformational change is not influenced by ionic strength. The addition of Qbeta RNA to the enzyme, does not result in the release of EF-Tu-Ts from the other enzyme subunits: whereas free EF-Tu-Ts binds GDP independently of salt concentration, this binding by Qbeta replicase is sensitive to high ionic strength and remains so in the presence of Qbeta RNA. Furthermore, RNA does not allow the release of EF-Ts from EF-Tu by GTP as measured by sensitivity of EF-Ts activity to N-ethylmaleimide.
Qβ复制酶是一种由四个不同亚基组成的复合物，很容易解离成两个亚复合物：30S核糖体蛋白S1和噬菌体编码的多肽（亚基I + II）以及蛋白质合成延伸因子EF-Tu和EF-Ts（亚基III + IV）。两个亚复合物之间的亲和力随着离子强度的增加而增加。该酶只有在低离子强度构象下才能利用合成模板起始RNA合成。起始多核苷酸链的延伸不受离子强度的影响。向酶中添加Qβ RNA也会改变其四级结构：在RNA存在的情况下，EF-Tu-Ts不能通过双功能交联剂与其他酶亚基共价连接。这种构象变化不受离子强度的影响。向酶中添加Qβ RNA不会导致EF-Tu-Ts从其他酶亚基上释放：游离的EF-Tu-Ts独立于盐浓度结合GDP，而Qβ复制酶的这种结合对高离子强度敏感，并且在Qβ RNA存在的情况下仍然如此。此外，如通过EF-Ts活性对N-乙基马来酰亚胺的敏感性所测定的，RNA不允许GTP使EF-Ts从EF-Tu上释放。
