Levine J G, Knasmüller S, Shelton M L, DeMarini D M
Department of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill.
Environ Mol Mutagen. 1994;24(1):11-22. doi: 10.1002/em.2850240104.
We used colony probe hybridization and PCR/DNA sequence analysis to determine the mutations in approximately 1,640 revertants of the -1 frameshift allele hisD3052 and approximately 260 revertants of the base substitution allele hisG46 of Salmonella typhimurium induced by the heterocyclic amine cooked food mutagen 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1). All of the mutations were at sites containing guanine, which is the base at which Glu-P-1 forms DNA adducts. A hotspot mutation involving the deletion of a CG or GC within the sequence CGCGCGCG accounted for 100% of the Glu-P-1-induced mutations at the frameshift allele in strains TA1978 (uvr+) and TA1538 (delta uvrB) and 99% in TA98 (delta uvrB, pKM101). To explain the induction of these hotspot mutations by Glu-P-1, we describe here a more detailed version of our recently proposed correct incorporation/slippage model [Genetics:136:731, 1994]. We propose that after cytosine is incorporated correctly opposite a Glu-P-1-adducted guanine, various slipped intermediates may form (a total of 18), depending on which guanine is adducted and whether it remains within the helix or becomes extrahelical. This variety of mutational pathways may account for the high mutability of the hotspot sequence by Glu-P-1. Although the pKM101 plasmid does not influence the mutagenic potency or mutational spectrum of Glu-P-1 at the frameshift allele, it is required by Glu-P-1 to revert the base substitution allele, where Glu-P-1 induces G-C --> T-A transversions (75%) and G-C --> tA-T transitions (25%) exclusively at a single site (the second position of the CCC codon of the hisG46 allele). The limited (20-30 times less) base substitution mutagenic potency of Glu-P-1 relative to its frameshift mutagenic potency as well as the extreme site specificity exhibited by Glu-P-1 for base substitutions may have bearing on the lack of base substitutions identified in ras genes in Glu-P-1-induced rat colon tumors.
我们运用菌落探针杂交以及聚合酶链式反应/DNA序列分析，来确定鼠伤寒沙门氏菌的-1移码等位基因hisD3052的约1640个回复突变体，以及碱基替换等位基因hisG46的约260个回复突变体中的突变情况。这些突变体是由杂环胺类烹饪食物诱变剂2-氨基-6-甲基二吡啶并[1,2-a:3',2'-d]咪唑（Glu-P-1）诱导产生的。所有突变均发生在含有鸟嘌呤的位点，而鸟嘌呤正是Glu-P-1形成DNA加合物的碱基。在序列CGCGCGCG中涉及缺失一个CG或GC的热点突变，在TA1978（uvr+）和TA1538（delta uvrB）菌株的移码等位基因中，占Glu-P-1诱导突变的100%，在TA98（delta uvrB，pKM101）中占99%。为了解释Glu-P-1对这些热点突变的诱导作用，我们在此描述一个比我们最近提出的正确掺入/滑链模型[《遗传学》：136：731，1994]更详细的版本。我们提出，在胞嘧啶正确掺入与Glu-P-1加合的鸟嘌呤相对的位置后，可能会形成各种滑链中间体（总共18种），这取决于哪个鸟嘌呤被加合，以及它是保留在螺旋内还是成为螺旋外的。这种多样的突变途径可能解释了Glu-P-1对热点序列的高诱变率。尽管pKM101质粒不影响Glu-P-1在移码等位基因处的诱变效力或突变谱，但Glu-P-1要使碱基替换等位基因回复突变则需要它。在碱基替换等位基因处，Glu-P-1仅在单个位点（hisG46等位基因CCC密码子的第二位）诱导G-C→T-A颠换（75%）和G-C→tA-T转换（25%）。Glu-P-1相对于其移码诱变效力而言有限的（少20 - 30倍）碱基替换诱变效力，以及Glu-P-1对碱基替换所表现出的极端位点特异性，可能与在Glu-P-1诱导的大鼠结肠肿瘤的ras基因中未发现碱基替换有关。
