Wood R D, Hutchinson F
J Mol Biol. 1984 Mar 5;173(3):293-305. doi: 10.1016/0022-2836(84)90122-0.
Non-targeted mutagenesis of lambda phage by ultraviolet light is the increase over background mutagenesis when non-irradiated phage are grown in irradiated Escherichia coli host cells. Such mutagenesis is caused by different processes from targeted mutagenesis, in which mutations in irradiated phage are correlated with photoproducts in the phage DNA. Non-irradiated phage grown in heavily irradiated uvr+ host cells showed non-targeted mutations, which were 3/4 frameshifts, whereas targeted mutations were 2/3 transitions. For non-targeted mutagenesis in heavily irradiated host cells, there were one to two mutant phage per mutant burst. From this and the pathways of lambda DNA synthesis, it can be argued that non-targeted mutagenesis involves a loss of fidelity in semiconservative DNA replication. A series of experiments with various mutant host cells showed a major pathway of non-targeted mutagenesis by ultraviolet light, which acts in addition to "SOS induction" (where cleavage of the LexA repressor by RecA protease leads to din gene induction): (1) the induction of mutants has the same dependence on irradiation for wild-type and for umuC host cells; (2) a strain in which the SOS pathway is constitutively induced requires irradiation to the same level as wild-type cells in order to fully activate non-targeted mutagenesis; (3) non-targeted mutagenesis occurs to some extent in irradiated recA recB cells. In cells with very low levels of PolI, the induction of non-targeted mutagenesis by ultraviolet light is enhanced. We propose that the major pathway for non-targeted mutagenesis in irradiated host cells involves binding of the enzyme DNA polymerase I to damaged genomic DNA, and that the low polymerase activity leads to frameshift mutations during semiconservative DNA replication. The data suggest that this process will play a much smaller role in ultraviolet mutagenesis of the bacterial genome than it does in the mutagenesis of lambda phage.
当未照射的噬菌体在经紫外线照射的大肠杆菌宿主细胞中生长时，λ噬菌体的非靶向诱变是指诱变率高于背景诱变率。这种诱变是由与靶向诱变不同的过程引起的，在靶向诱变中，受照射噬菌体中的突变与噬菌体DNA中的光产物相关。在经大量照射的uvr⁺宿主细胞中生长的未照射噬菌体显示出非靶向突变，其中3/4是移码突变，而靶向突变则是2/3是转换突变。对于在大量照射的宿主细胞中的非靶向诱变，每个突变爆发中有一到两个突变噬菌体。基于此以及λDNA合成途径，可以认为非靶向诱变涉及半保留DNA复制过程中保真度的丧失。一系列针对各种突变宿主细胞的实验表明，紫外线引发非靶向诱变的主要途径除了“ SOS诱导”（RecA蛋白酶切割LexA阻遏物导致din基因诱导）之外，还具有以下作用：（1）野生型和umuC宿主细胞的突变体诱导对辐射的依赖性相同；（2）SOS途径组成型诱导的菌株需要与野生型细胞接受相同水平的照射，以充分激活非靶向诱变；（3）在受照射的recA recB细胞中，非靶向诱变在一定程度上会发生。在PolI水平非常低的细胞中，紫外线诱导非靶向诱变的作用会增强。我们认为，受照射宿主细胞中非靶向诱变的主要途径涉及DNA聚合酶I与受损基因组DNA的结合，并且低聚合酶活性会导致半保留DNA复制过程中出现移码突变。数据表明，与在λ噬菌体诱变中相比，该过程在细菌基因组的紫外线诱变中所起的作用要小得多。
