Sargentini N J, Smith K C
Department of Microbiology and Immunology, Kirksville College of Osteopathic Medicine, MO 63501.
Mutat Res. 1994 Sep 1;309(2):147-63. doi: 10.1016/0027-5107(94)90088-4.
An extensive spectrum of ionizing radiation mutagenesis was determined by sequencing 318 137Cs gamma-radiation (anoxic)-induced episomal lacId mutations in Escherichia coli strain NR9102. The most commonly found radiation-induced mutations were base substitutions (44% transversions and 41% transitions). The radiation-induced spectrum consisted of: 23% G.C-->A.T, 18% A.T-->G.C, 17% G.C-->T.A, 14% G.C-->C.G, 8% A.T-->T.A, 6% A.T-->C.G, 8% single-base deletions, 5% multiple mutations, 3% multi-base deletions, and essentially no single- or multi-base additions. This spectrum compared better with spectra for other systems obtained by in vivo irradiation than with one obtained by in vitro irradiation. Multiple mutations, which were unique to the radiation-induced spectrum, generally consisted of one active and one closely linked silent mutation, and are suggested to result from an altered replication complex of reduced fidelity. Mutation rates were 4.1 x 10(-8) lac-constitutive mutations/gene/Gy and 1.2 x 10(-10) base substitutions/base pair/Gy. Thirty-two percent more radiation-induced mutations occurred at G.C vs. A.T base pairs. A strand asymmetry was noted for G.C-->C.G and A.T-->T.A transversions. A nearest-neighbor analysis showed that C (vs. A, G, or T), on either side of the mutation site, substantially enhanced most types of base substitutions. Similarly, G and C flanked both sides of single-base deletion sites twice as frequently as would be expected from the base composition of the mutation target. For comparative purposes, we sequenced 411 spontaneous lac-constitutive mutants of which 269 were lacId mutants, and there was good agreement between these and previously published mutational spectra. The spontaneous and radiation-induced mutational spectra differed substantially for virtually every class of mutation. For example, the set of spontaneous dominant lac-constitutive mutations contained many more mutations that did not map in the normal region for lacId mutations (i.e., 35% vs. 3%) and were presumed to be lacO-constitutive mutations. A sampling of these presumptive lacOc mutations was also sequenced: 17/22 (spontaneous) and 1/9 (radiation) were found to be lacOc long deletions, one from each set were base substitutions, and the remaining mutations showed the wild-type lacO sequence. Like the radiation-induced spectrum, the spontaneous spectrum showed enhanced mutagenesis at G.C sites, strand asymmetry, and enhanced mutagenesis when G or C were the nearest neighbors.
通过对大肠杆菌NR9102菌株中318个经137Csγ辐射（缺氧条件下）诱导的附加型lacId突变进行测序，确定了广泛的电离辐射诱变谱。最常见的辐射诱导突变是碱基替换（44%的颠换和41%的转换）。辐射诱导谱包括：23%的G.C→A.T、18%的A.T→G.C、17%的G.C→T.A、14%的G.C→C.G、8%的A.T→T.A、6%的A.T→C.G、8%的单碱基缺失、5%的多重突变、3%的多碱基缺失，基本上没有单碱基或多碱基插入。与通过体内照射获得的其他系统的谱相比，该谱与通过体外照射获得的谱更相似。多重突变是辐射诱导谱所特有的，通常由一个活性突变和一个紧密连锁的沉默突变组成，推测是由于保真度降低的复制复合体改变所致。突变率为4.1×10−8个乳糖组成型突变/基因/戈瑞和1.2×10−10个碱基替换/碱基对/戈瑞。在G.C碱基对处发生的辐射诱导突变比在A.T碱基对处多32%。观察到G.C→C.G和A.T→T.A颠换存在链不对称性。最近邻分析表明，突变位点两侧的C（与A、G或T相比）显著增强了大多数类型的碱基替换。同样，单碱基缺失位点两侧G和C出现的频率是突变靶点碱基组成预期频率的两倍。为了进行比较，我们对411个自发乳糖组成型突变体进行了测序，其中269个是lacId突变体，这些结果与先前发表的突变谱有很好的一致性。自发突变谱和辐射诱导突变谱在几乎每一类突变中都有很大差异。例如，自发显性乳糖组成型突变集中包含更多不在lacId突变正常区域定位的突变（即35%对3%），推测是lacO组成型突变。对这些推测的lacOc突变进行了抽样测序：22个自发突变体中有17个（自发）和9个辐射诱导突变体中有1个被发现是lacOc长缺失，每组各有一个是碱基替换，其余突变显示野生型lacO序列。与辐射诱导谱一样，自发谱在G.C位点显示诱变增强、链不对称性，并且当G或C是最近邻时诱变增强。
