Shukla R, Geacintov N E, Loechler E L
Department of Biology, Boston University, MA 02215, USA.
Carcinogenesis. 1999 Feb;20(2):261-8. doi: 10.1093/carcin/20.2.261.
Previously, in a random mutagenesis study, the (+)-anti diol epoxide of benzo[a]pyrene [(+)-anti-B[a]PDE] was shown to induce a complex mutational spectrum in the supF gene of an Escherichia coli plasmid, which included insertions, deletions and base substitution mutations, notably a significant fraction of GC-->TA, GC-->AT and GC-->CG mutations. At some sites, a single type of mutation dominated and to understand individual mutagenic pathways these sites were chosen for study by site-specific means to determine whether the major adduct, [+ta]-B[a]P-N2-dG, was responsible. [+ta]-B[a]P-N2-dG was shown to induce approximately 95% G-->T mutations in a 5'-TGC-3' sequence context and approximately 80% G-->A mutations in a 5'-CGT-3' sequence context. (+)-anti-B[a]PDE induced principally GC-->CG mutations in the G133 sequence context (5'-AGA-3') in studies using both SOS-uninduced or SOS-induced E. coli. Herein, [+ta]-B[a]P-N2-dG is shown to induce principally G-->A mutations (>90%) either without or with SOS induction in a closely related 5'-AGA-3' sequence context (identical over 7 bp). This is the first time that there has been a discrepancy between the mutagenic specificity of (+)-anti-B[a]PDE versus [+ta]-B[a]P-N2-dG. Eight explanations for this discordance are considered. Four are ruled out; e.g. the second most prevalent adduct [+ca]-B[a]P-N2-dG also induces a preponderance of G-->A mutations (>90%), so it also is not responsible for (+)-anti-B[a]PDE-induced G133-->C mutations. The four explanations not ruled out are discussed and include that another minor adduct might be responsible and that the 5'-AGA-3' sequence context differed slightly in the studies with [+ta]-B[a]P-N2-dG versus (+)-anti-B[a]PDE. In spite of the discordance, [+ta]-B[a]P-N2-dG induces G-->A mutations in the context studied herein and this result has proven useful in generating a hypothesis for what conformations of [+ta]-B[a]P-N2-dG are responsible for G-->T versus G-->A mutations.
此前，在一项随机诱变研究中，苯并[a]芘的(+)-反式二醇环氧化物[(+)-反式-B[a]PDE]在大肠杆菌质粒的supF基因中诱导出复杂的突变谱，其中包括插入、缺失和碱基替换突变，尤其是相当一部分GC→TA、GC→AT和GC→CG突变。在某些位点，单一类型的突变占主导，为了解单个诱变途径，选择这些位点通过位点特异性方法进行研究，以确定主要加合物[+ta]-B[a]P-N2-dG是否起作用。结果表明，[+ta]-B[a]P-N2-dG在5'-TGC-3'序列背景下诱导约95%的G→T突变，在5'-CGT-3'序列背景下诱导约80%的G→A突变。在使用未诱导SOS或诱导SOS的大肠杆菌的研究中，(+)-反式-B[a]PDE在G133序列背景(5'-AGA-3')中主要诱导GC→CG突变。在此，研究表明，[+ta]-B[a]P-N2-dG在紧密相关的5'-AGA-3'序列背景(7个碱基以上相同)中，无论有无SOS诱导，主要诱导G→A突变(>90%)。这是(+)-反式-B[a]PDE与[+ta]-B[a]P-N2-dG的诱变特异性首次出现差异。文中考虑了对此不一致的八种解释。四种被排除；例如，第二常见的加合物[+ca]-B[a]P-N2-dG也诱导大量的G→A突变(>90%)，所以它也不是(+)-反式-B[a]PDE诱导的G133→C突变的原因。对未被排除的四种解释进行了讨论，包括可能是另一种次要加合物起作用，以及在[+ta]-B[a]P-N2-dG与(+)-反式-B[a]PDE的研究中，5'-AGA-3'序列背景略有不同。尽管存在不一致，但[+ta]-B[a]P-N2-dG在本文研究的背景下诱导G→A突变，这一结果已被证明有助于提出关于[+ta]-B[a]P-N2-dG的哪些构象导致G→T与G→A突变的假说。
