Lawson C L, Sigler P B
Department of Biochemistry and Molecular Biology, University of Chicago, Illinois 60637.
Nature. 1988 Jun 30;333(6176):869-71. doi: 10.1038/333869a0.
The trp repressor is a small dimeric regulatory protein which controls the expression of three operons in Escherichia coli. The inactive aporepressor protein must bind two molecules of L-tryptophan to form the active repressor. If desamino analogues of L-tryptophan such as indole propionate (IPA) are substituted for L-tryptophan, an inactive pseudorepressor is formed. Because the desamino analogues thus cause derepression of operons under control of the trp repressor, they appear to be 'inducers'. We have determined the crystal structure of the pseudorepressor and refined it to 1.65 A. The molecular structure was compared to that of the nearly isomorphous orthorhombic form of the repressor. Surprisingly, the indole ring of IPA is in the same position as the indole ring of L-tryptophan in the repressor, but is 'flipped over'. As a result, the carboxyl group of IPA is oriented toward the DNA-binding surface of the protein and is in a position where it sterically and electrostatically repels the phosphate backbone of both operator and non-operator DNA. This explains why IPA acts as an apparent trp inducer.
色氨酸阻遏物是一种小的二聚体调节蛋白，它控制大肠杆菌中三个操纵子的表达。无活性的脱辅基阻遏蛋白必须结合两分子L-色氨酸才能形成活性阻遏物。如果用L-色氨酸的脱氨基类似物如吲哚丙酸（IPA）替代L-色氨酸，就会形成无活性的假阻遏物。由于这些脱氨基类似物会导致色氨酸阻遏物控制下的操纵子去阻遏，它们似乎是“诱导物”。我们已经确定了假阻遏物的晶体结构并将其精修至1.65 Å。将该分子结构与阻遏物的近同晶正交形式的结构进行了比较。令人惊讶的是，IPA的吲哚环与阻遏物中L-色氨酸的吲哚环处于相同位置，但“翻转了”。结果，IPA的羧基朝向蛋白质的DNA结合表面，并且处于在空间上和静电上排斥操纵基因和非操纵基因DNA的磷酸骨架的位置。这就解释了为什么IPA作为一种明显的色氨酸诱导物起作用。
