Friedman A M, Fischmann T O, Steitz T A
Department of Molecular Biophysics and Biochemistry, Howard Hughes Medical Institute, Yale University, New Haven, CT 06520-8114, USA.
Science. 1995 Jun 23;268(5218):1721-7. doi: 10.1126/science.7792597.
The crystal structure of the tryptic core fragment of the lac repressor of Escherichia coli (LacR) complexed with the inducer isopropyl-beta-D-thiogalactoside was determined at 2.6 A resolution. The quaternary structure consists of two dyad-symmetric dimers that are nearly parallel to each other. This structure places all four DNA binding domains of intact LacR on the same side of the tetramer, and results in a deep, V-shaped cleft between the two dimers. Each monomer contributes a carboxyl-terminal helix to an antiparallel four-helix bundle that functions as a tetramerization domain. Some of the side chains whose mutation reduce DNA binding form clusters on a surface near the amino terminus. Placing the structure of the DNA binding domain complexed with operator previously determined by nuclear magnetic resonance onto this surface results in two operators being adjacent and nearly parallel to each other. Structural considerations suggest that the two dimers of LacR may flexibly alter their relative orientation in order to bind to the known varied spacings between two operators.
测定了与诱导剂异丙基-β-D-硫代半乳糖苷复合的大肠杆菌乳糖阻遏物（LacR）胰蛋白酶核心片段的晶体结构，分辨率为2.6埃。四级结构由两个近乎相互平行的二轴对称二聚体组成。这种结构使完整LacR的所有四个DNA结合结构域位于四聚体的同一侧，并在两个二聚体之间形成一个深的V形裂隙。每个单体为一个反平行四螺旋束贡献一个羧基末端螺旋，该四螺旋束作为四聚化结构域起作用。一些突变会降低DNA结合能力的侧链在氨基末端附近的一个表面上形成簇。将先前通过核磁共振确定的与操纵基因复合的DNA结合结构域的结构放置在这个表面上，会导致两个操纵基因相邻且近乎相互平行。结构上的考虑表明，LacR的两个二聚体可能会灵活改变它们的相对取向，以便与两个操纵基因之间已知的不同间距结合。
