Takeda Y, Folkmanis A, Echols H
J Biol Chem. 1977 Sep 10;252(17):6177-83.
The Cro protein specified by bacteriophage lambda is a repressor of the genes expressed early in phage development and is required for a normal late stage of lytic growth. We have purified Cro protein to virtual homogeneity and analyzed its structure and properties as a DNA-binding protein and repressor of RNA synthesis. To confirm that the protein is the product of the cro gene, we have also shown that a missense mutation in the cro gene leads to a product that is more temperature- and salt-sensitive in its DNA-binding property. As purified, Cro protein is a dimer of identical subunits of molecular weight 8600. The purified protein binds to lambda-DNA carrying the specific binding sites (operators oL and oR) with an estimated dissociation constant of 10(-10) M to 10(-11) M; there is also weaker binding to other sites on DNA, as found for other DNA-binding regulatory proteins. In a purified transcription system, the Cro protein is an effective and specific repressor of RNA synthesis from the N and cro genes; thus Cro is an autorepressor which regulates its own synthesis. A comparison of the properties of the two lambda repressor proteins, cI and Cro, indicates that cI is a "strong repressor" specialized for complete turnoff of lytic functions needed for the maintenance of lysogeny, whereas Cro is a "weak repressor" specialized for a gradual turnoff of early viral genes that potentiates the late stage of lytic development.
λ噬菌体编码的Cro蛋白是噬菌体发育早期所表达基因的阻遏物，也是溶菌生长正常后期所必需的。我们已将Cro蛋白纯化至几乎同质，并分析了其作为DNA结合蛋白和RNA合成阻遏物的结构与特性。为证实该蛋白是cro基因的产物，我们还表明cro基因中的一个错义突变会导致一种在DNA结合特性上对温度和盐更敏感的产物。纯化后的Cro蛋白是由分子量为8600的相同亚基组成的二聚体。纯化后的蛋白与携带特定结合位点（操纵子oL和oR）的λ-DNA结合，估计解离常数为10^(-10) M至10^(-11) M；与DNA上的其他位点也有较弱的结合，这与其他DNA结合调节蛋白的情况相同。在一个纯化的转录系统中，Cro蛋白是N基因和cro基因RNA合成的有效且特异性的阻遏物；因此，Cro是一种自我调节其自身合成的自阻遏物。对两种λ阻遏蛋白cI和Cro的特性比较表明，cI是一种“强阻遏物”，专门用于完全关闭维持溶原性所需的溶菌功能，而Cro是一种“弱阻遏物”，专门用于逐渐关闭早期病毒基因，从而促进溶菌发育的后期阶段。
