Takahashi M, Blazy B, Baudras A
Nucleic Acids Res. 1979 Nov 24;7(6):1699-712. doi: 10.1093/nar/7.6.1699.
The cyclic adenosine 3',5'-monophosphate receptor protein of Escherichia coli (CRP) binds cooperatively to single- and double-stranded DNA. Binding data could be fitted to the model of McGhee and von Hippel (1) and show that neither strandedness of DNA, nor the effectors cAMP and cGMP or the ionic strength (KCl) do change appreciably the cooperativity parameter omega (omega approximately or equal to 100), and site size of DNA. Instead, distinctly different slopes were observed for the linear decrease of log K omega (a measure of the overall affinity) as a function of log (K+). From these double-log plots (2), the number of cations released and the non-electrostatic contributions to the binding free energy could be determined. Binding of CRP to single-stranded DNA is slightly favored under physiological ionic conditions (0.15-0.20 M), but such a preferential binding is almost abolished in the presence of cAMP which increases the strength of the interaction of the protein with both forms of DNA. CGMP does not change the binding properties and interactions of CRP with DNA. These observations do not support the proposal that the cAMP-CRP complex could stimulate transcription via some "melting" property unless its interactions be dramatically changed when it binds specifically to promoter DNA.
大肠杆菌的环腺苷酸3',5'-单磷酸受体蛋白（CRP）能与单链和双链DNA协同结合。结合数据可拟合到McGhee和von Hippel（1）的模型中，结果表明，DNA的链性、效应分子环磷酸腺苷（cAMP）和环磷酸鸟苷（cGMP）以及离子强度（KCl）均不会显著改变协同参数ω（ω≈100）和DNA的位点大小。相反，观察到log Kω（衡量总体亲和力的指标）随log（K⁺）呈线性下降时，斜率明显不同。从这些双对数图（2）中，可以确定释放的阳离子数量以及对结合自由能的非静电贡献。在生理离子条件（0.15 - 0.20 M）下，CRP与单链DNA的结合略占优势，但在存在cAMP的情况下，这种优先结合几乎被消除，cAMP会增加蛋白质与两种形式DNA相互作用的强度。cGMP不会改变CRP与DNA的结合特性和相互作用。这些观察结果不支持以下观点：除非cAMP - CRP复合物在特异性结合启动子DNA时其相互作用发生显著变化，否则它可以通过某种“解链”特性刺激转录。
