Levin-Zaidman S, Reich Z, Wachtel E J, Minsky A
Department of Organic Chemistry, The Weizmann Institute of Science, Israel.
Biochemistry. 1996 Mar 5;35(9):2985-91. doi: 10.1021/bi9525482.
Closed-circular supercoiled DNA molecules have been shown to form a cholesteric assembly within bacteria as well as in vitro under physiological DNA and salt concentrations. Circular dichroism and X-ray scattering studies indicate that the macroscopic structural properties of the chiral mesophase are directly and uniquely dictated by the supercoiling parameters of the constituent molecules. Specifically, we find that the pitch of the DNA cholesteric phase derived from supercoiled DNA is determined by the superhelical density, which, in turn, is modulated by secondary conformational changes. A direct interrelationship among four DNA structural levels, namely, DNA sequence, secondary structural transitions, the tertiary superhelical conformation, and the quaternary, supramolecular organization is accordingly pointed out. Since secondary conformational changes are both sequence and environment dependent, alterations of cellular conditions may effectively modulate the properties of the packed DNA organization, through their effects on secondary structural transitions and hence on the superhelical parameters. On the basis of these results we suggest that liquid crystallinity represents an effectively regulated packaging mode of plectonemic, nucleosome-free DNA molecules in living systems.
已表明，在生理DNA和盐浓度下，闭环超螺旋DNA分子在细菌内以及体外均能形成胆甾相聚集体。圆二色性和X射线散射研究表明，手性中间相的宏观结构性质直接且唯一地由组成分子的超螺旋参数决定。具体而言，我们发现由超螺旋DNA衍生的DNA胆甾相的螺距由超螺旋密度决定，而超螺旋密度又受二级构象变化的调节。因此指出了DNA的四个结构水平之间的直接相互关系，即DNA序列、二级结构转变、三级超螺旋构象和四级超分子组织。由于二级构象变化既取决于序列又取决于环境，细胞条件的改变可能通过影响二级结构转变进而影响超螺旋参数，从而有效地调节堆积DNA组织的性质。基于这些结果，我们认为液晶性代表了活系统中麻花状、无核小体DNA分子的一种有效调控的包装模式。
