Howard-Flanders P
Basic Life Sci. 1975;5A:265-74. doi: 10.1007/978-1-4684-2895-7_35.
DNA molecules that have been damaged in both strands at the same level are not subject to repair by excision but instead can be repaired through recombination with homologous molecules. Examples of two-strand damage include postreplication gaps opposite pyrimidine dimers, two-strand breaks produced by X-rays, and chemically induced interstrand cross-links. In ultraviolet-irradiated bacteria, the newly synthesized DNA is of length equal to the interdimer spacing. With continued incubation, this low-molecular-weight DNA is joined into high-molecular-weight chains (postreplication repair), a process associated with sister exchanges in bacteria. Recombination is initiated by pyrimidine dimers opposite postreplication gaps and by interstrand cross-links that have been cut by excision enzymes. The free ends at the resulting gaps presumably initiate the exchanges. Postreplication repair in Escherichia coli occurs in recB- AND RECC but is greatly slowed in recF- mutants. RecB and recC are the structural genes for exonuclease V, which digests two-stranded DNA by releasing oligonucleotides first from one strand and then from the other. The postreplication sister exchanges in ultra-violet-irradiated bacteria result in the distribution of pyrimidine dimers between parental and daughter strands, indicating that long exchanges involving both strands of each duplex occur. The R1 restriction endonuclease from E. COli has been used to cut the DNA of a bacterial drug-resistance transfer factor with one nuclease-sensitive site, and also DNA from the frog Xenopus enriched for ribosomal 18S and 28S genes. The fragments were annealed with the cut plasmid DNA and ligated, producing a new larger plasmid carrying the eukaryotic rDNA and able to infect and replicate in E. coli.
在同一水平上两条链都受损的DNA分子不能通过切除进行修复，而是可以通过与同源分子重组来修复。双链损伤的例子包括嘧啶二聚体对面的复制后缺口、X射线产生的双链断裂以及化学诱导的链间交联。在紫外线照射的细菌中，新合成的DNA长度等于二聚体间距。持续孵育后，这种低分子量DNA会连接成高分子量链（复制后修复），这一过程与细菌中的姐妹交换有关。重组由复制后缺口对面的嘧啶二聚体以及被切除酶切割的链间交联引发。由此产生的缺口处的自由端可能引发交换。大肠杆菌中的复制后修复发生在recB和recC基因存在时，但在recF突变体中会大大减慢。RecB和recC是外切核酸酶V的结构基因，外切核酸酶V通过先从一条链然后从另一条链释放寡核苷酸来消化双链DNA。紫外线照射的细菌中的复制后姐妹交换导致嘧啶二聚体在亲代链和子代链之间分布，这表明涉及每个双链体两条链的长交换发生。来自大肠杆菌的R1限制性内切酶已被用于切割具有一个核酸酶敏感位点的细菌抗药转移因子的DNA，以及来自非洲爪蟾的富含核糖体18S和28S基因的DNA。这些片段与切割后的质粒DNA退火并连接，产生一个携带真核rDNA且能够在大肠杆菌中感染和复制的新的更大质粒。
