Levine J G, Schaaper R M, DeMarini D M
Department of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill 27599.
Genetics. 1994 Mar;136(3):731-46. doi: 10.1093/genetics/136.3.731.
We used colony probe hybridization and polymerase chain reaction/DNA sequence analysis to determine the mutations in approximately 2,400 4-aminobiphenyl (4-AB) +S9-induced revertants of the -1 frameshift allele hisD3052 and of the base-substitution allele hisG46 of Salmonella typhimurium. Most of the mutations occurred at sites containing guanine, which is the primary base at which 4-AB forms DNA adducts. A hotspot mutation involving the deletion of a CG or GC within the sequence CGCGCGCG accounted for 100 and 99.9%, respectively, of the reversion events at the hisD3052 allele in the pKM101 plasmid-minus strains TA1978 (uvr+) and TA1538 (delta uvrB). In strain TA98 (delta uvrB, pKM101), which contained the SOS DNA repair system provided by the pKM101 plasmid, approximately 85% of the revertants also contained the hotspot deletion; the remaining approximately 15% contained one of two types of mutations: (1) complex frameshifts that can be described as a -2 or +1 frameshift and an associated base substitution and (2) deletions of the CC or GG sequences that flank the hotspot site (CCGCGCGCGG). We propose a misincorporation/slippage model to account for these mutations in which (1) pKM101-mediated misincorporation and translesion synthesis occurs across a 4-AB-adducted guanine; (2) the instability of such a mispairing and/or the presence of the adduct leads to strand slippage in a run of repeated bases adjacent to the adducted guanine; and (3) continued DNA synthesis from the slipped intermediate produces a frameshift associated with a base substitution. This model readily accounts for the deletion of the CC or GG sequences flanking the hotspot site, indicating that these mutations are, in fact, complex mutations in disguise (i.e., cryptic complex frameshifts). The inferred base-substitution specificity associated with the complex frameshifts at the hisD3052 allele (primarily G.C-->T.A transversions) is consistent with the finding that 4-AB induced primarily G.C-->T.A transversions at the hisG46 base-substitution allele. The model also provides a framework for understanding the different relative mutagenic potencies of 4-AB at the two alleles in the various DNA repair backgrounds of Salmonella.
我们使用菌落探针杂交和聚合酶链反应/DNA序列分析来确定在约2400个由4-氨基联苯（4-AB）+S9诱导的鼠伤寒沙门氏菌-1移码等位基因hisD3052和碱基置换等位基因hisG46的回复突变体中的突变。大多数突变发生在含有鸟嘌呤的位点，鸟嘌呤是4-AB形成DNA加合物的主要碱基。涉及在序列CGCGCGCG内缺失一个CG或GC的热点突变，分别占pKM101质粒缺失菌株TA1978（uvr +）和TA1538（δuvrB）中hisD3052等位基因回复事件的100%和99.9%。在含有由pKM101质粒提供的SOS DNA修复系统的TA98（δuvrB，pKM101）菌株中，约85%的回复突变体也含有热点缺失；其余约15%含有两种类型突变之一：（1）可描述为-2或+1移码及相关碱基置换的复杂移码，以及（2）热点位点（CCGCGCGCGG）侧翼的CC或GG序列的缺失。我们提出一个错配掺入/滑动模型来解释这些突变，其中（1）pKM101介导的错配掺入和跨损伤合成发生在4-AB加合的鸟嘌呤上；（2）这种错配的不稳定性和/或加合物的存在导致在加合鸟嘌呤相邻的一段重复碱基中发生链滑动；（3）从滑动中间体继续进行DNA合成产生与碱基置换相关的移码。该模型很容易解释热点位点侧翼的CC或GG序列的缺失，表明这些突变实际上是伪装的复杂突变（即隐蔽的复杂移码）。在hisD3052等位基因处与复杂移码相关的推断碱基置换特异性（主要是G.C→T.A颠换）与4-AB在hisG46碱基置换等位基因处主要诱导G.C→T.A颠换的发现一致。该模型还为理解4-AB在鼠伤寒沙门氏菌各种DNA修复背景下在两个等位基因处不同的相对诱变潜能提供了一个框架。
