ORIGIN OF ULTRAVIOLET DAMAGE IN DNA
ORIGIN OF ULTRAVIOLET DAMAGE IN DNA
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DOI：
10.1016/0022-2836(89)90120-4
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发表时间：
1989-12-05
影响因子：
5.6
通讯作者：
WANG, Z
中科院分区：
文献类型：
--
作者：
BECKER, MM;WANG, Z
一种新型的紫外（u.v.）足迹技术已被用于分析在基因为双链或单链的条件下，在5S rRNA基因的250个碱基处紫外光产物的形成。由于紫外线足迹技术可以检测到比以前更多类型的紫外线损伤，我们已经能够详细研究为什么DNA中的某些碱基会被紫外线损伤，而其他碱基则不会。我们的测量结果表明，紫外光损伤DNA中给定碱基的能力由两个因素决定，即光产物附近的DNA序列和光产物位点处DNA的柔性。对于嘧啶，双链DNA中的主要光反应涉及相邻嘧啶残基之间的共价二聚化。二聚化在解链的DNA中更容易，因为相邻嘧啶残基二聚化所需的几何变化在单链DNA中更容易。紫外光子的吸收不能同时引起相邻嘧啶或其它碱基彼此二聚化所需的几何变化。相反，在吸收u.v.光子时，只有那些在激发期间能够容易地形成光二聚体的几何形状中的热激发碱基可以发生光反应。与相邻的嘧啶相反，非相邻的嘧啶（两侧都有嘌呤的嘧啶）不容易在双链DNA中形成u.v.光产物。由于在非相邻嘧啶残基的光反应在单链DNA中大大增强，它们在双螺旋DNA中未能形成归因于双螺旋所施加的扭转约束，这使得非相邻嘧啶难以采用光反应所需的几何形状。虽然嘌呤被认为是抗紫外线损伤的，但我们的测量表明，在中等紫外线剂量下，在其5“侧被两个或多个相邻嘧啶侧接的嘌呤容易在双链DNA中形成紫外线光产物。侧翼嘧啶似乎通过将三重激发能量转移到嘌呤来激活嘌呤光反应。DNA螺旋的熔化极大地抑制了侧翼嘧啶激活嘌呤光反应的能力，推测是通过破坏三重态转移所需的紧密轨道重叠。
A novel ultraviolet (u.v.) footprinting technique has been used to analyze the formation of u.v. photoproducts at 250 bases of a 5 S rRNA gene under conditions where the gene is either double or single-stranded. Because many more types of u.v. damage can be detected by the u.v. footprinting technique than has been previously possible, we have been able to examine in detail why certain bases in DNA are damaged by u.v. light while others are not. Our measurements demonstrate that the ability of u.v. light to damage a given base in DNA is determined by two factors, the sequence of the DNA in the immediate vicinity of the photoproduct, and the flexibility of the DNA at the site of the photoproduct. For pyrimidines, the predominant photoreaction in double-stranded DNA involves covalent dimerization between adjacent pyrimidine residues. Dimerization is much easier in melted DNA because the geometrical changes required for adjacent pyrimidine residues to dimerize are easier in single-stranded DNA. The absorption of a u.v. photon cannot simultaneously induce the geometrical changes required for adjacent pyrimidines or other bases to dimerize with one another. Rather, upon the absorption of a u.v. photon, only those thermally excited bases that are in a geometry capable of easily forming a photodimer during excitation, can photoreact. In contrast to adjacent pyrimidines, non-adjacent pyrmidines (pyrimidines flanked on either side by a purine) do not readily form u.v. photoproducts in double-stranded DNA. Because photoreactions at non-adjacent pyrimidien residues are greatly enhanced in single-stranded DNA, their failure to form in double-helical DNA is attributed to torsional constraints imposed by the double helix which make it difficult for non-adjacent pyrimidines to adopt a geometry necessary for photoreaction. Although purines are belived to be resistant to u.v. damage, our measurments demonstrate that at moderate u.v. dosages purines which are flanked on their 5'' side by two or more contiguous pyrimidines readily form u.v. photoproducts in double-stranded DNA. Flanking pyrimidines appear to activate purine photoreactions by transferring triplet excitation energy to the purine. Melting of the DNA helix greatly inhibits the ability of flanking pyrimidines to activate purine photoreactions, presumably by disrupting intimate orbital overlap requred for triplet transfer.