Ellison M J, Feigon J, Kelleher R J, Wang A H, Habener J F, Rich A
Biochemistry. 1986 Jun 17;25(12):3648-55. doi: 10.1021/bi00360a026.
The ability of a stretch of alternating dA-dT to adopt the left-handed Z form has been assessed by examining the behavior of the sequence d(CG)6(TA)4(CG)6 contained in the plasmid pBR322. The structural transition occurring within this sequence as a function of negative superhelicity was analyzed by several methods, including (1) the supercoiling-dependent unwinding of the insert as determined by two-dimensional gel electrophoresis, (2) the binding of anti-Z-DNA antibodies to the insert, (3) the sensitivity of the sequence to a single strand specific endonuclease, and (4) the sensitivity of the insert to digestion by a restriction endonuclease that cuts within the d(CG)6 segments when in the right-handed form. These studies have shown that in negatively supercoiled DNA the two d(CG)6 portions of the insert adopt the Z form, while the central d(TA)4 segment forms an underwound structure with a helical repeat that is best approximated as being intermediate between the B form and the Z form. A statistical mechanical treatment of the unwinding of the insert as a function of negative superhelicity provides an estimate of the minimum free energy required to convert an A-T bp from the B form to the Z form, as well as the free energy associated with the conversion of an A-T bp from the B form to the unwound form. These results strongly indicate that Z DNA is an unfavored structural alternative for stretches of d(AT)n in negatively supercoiled DNA.
通过检测质粒pBR322中包含的序列d(CG)6(TA)4(CG)6的行为，评估了一段交替的dA-dT形成左手Z型的能力。采用多种方法分析了该序列内随着负超螺旋度变化而发生的结构转变，包括：(1) 通过二维凝胶电泳确定插入片段的超螺旋依赖性解旋；(2) 抗Z-DNA抗体与插入片段的结合；(3) 该序列对单链特异性核酸内切酶的敏感性；(4) 当处于右手形式时，插入片段对在d(CG)6片段内切割的限制性核酸内切酶消化的敏感性。这些研究表明，在负超螺旋DNA中，插入片段的两个d(CG)6部分形成Z型，而中间的d(TA)4片段形成一种解旋结构，其螺旋重复最接近B型和Z型之间的中间状态。对插入片段作为负超螺旋度函数的解旋进行统计力学处理，可估算将一个A-T碱基对从B型转变为Z型所需的最小自由能，以及与将一个A-T碱基对从B型转变为解旋形式相关的自由能。这些结果有力地表明，对于负超螺旋DNA中的d(AT)n片段，Z-DNA是一种不利的结构变体。
