lac repressor forms loops with linear DNA carrying two suitably spaced lac operators.
lac repressor forms loops with linear DNA carrying two suitably spaced lac operators.
复制标题
lac阻遏物与携带两个适当间隔的lac操纵子的线性DNA形成环。
作者：
Helmut Kramer;Monika Niemoller;Michele Amouyall;Bernard Revet;B. Wilcken;B. Muller
四聚体lac阻遏物可以与一个DNA片段上的两个lac操纵子结合，并诱导插入的DNA形成环。电子显微镜、非变性聚丙烯酰胺凝胶电泳和DNA酶I保护实验被用来证明这样的DNA环，其中两个lac操纵基因的对称中心之间的距离在63和535 bp之间变化。DNA环的形成有利于两个乳糖操纵子的正确定相和反应的两种组分的低浓度。当使用大量过量的乳糖阻遏物超过DNA时，观察到“串联”结构，其中两个乳糖操纵子独立地被两个阻遏物四聚体占据。当lac阻遏子和lac操纵子的浓度都很高时，观察到一种“三明治”结构，其中两个DNA分子由两个lac阻遏子四聚体反式连接。
Tetrameric lac repressor may bind to two lac operators on one DNA fragment and induce the intervening DNA to form a loop. Electron microscopy, non‐denaturing polyacrylamide gel electrophoresis, and DNase I protection experiments were used to demonstrate such DNA loops, where the distance between the centres of symmetry of the two lac operators varies between 63 and 535 bp. Formation of a DNA loop is favoured by correct phasing of the two lac operators and a low concentration of both components of the reaction. When a large excess of lac repressor over DNA is used, a ‘tandem’ structure is observed, in which both lac operators are occupied independently by two repressor tetramers. When the concentrations of both lac repressor and lac operator are high, a ‘sandwich’ structure is observed, in which two DNA molecules are connected by two lac repressor tetramers in trans.