title: Comparative mutagenesis of the C8-guanine adducts of 1-nitropyrene and 1,6- and 1,8-dinitropyrene in a CpG repeat sequence. A slipped frameshift intermediate model for dinucleotide deletion.
abstract: In the Ames Salmonella typhimurium reversion assay 1,6- and 1,8-dinitropyrenes (1,6- and 1,8-DNPs) are much more potent mutagens than 1-nitropyrene (1-NP). Genetic experiments established that certain differences in the metabolism of the DNPs, which in turn result in increased DNA adduction, play a role. It remained unclear, however, if the DNP adducts, N-(guanin-8-yl)-1-amino-6 ()-nitropyrene (Gua-C8-1,6-ANP and Gua-C8-1,8-ANP), which contain a nitro group on the pyrene ring covalently linked to the guanine C8, are more mutagenic than the major 1-NP adduct, N-(guanin-8-yl)-1-aminopyrene (Gua-C8-AP). In order to address this, we have compared the mutation frequency of the three guanine C8 adducts, Gua-C8-AP, Gua-C8-1,6-ANP, and Gua-C8-1,8-ANP in a CGCG*CG sequence. Single-stranded M13mp7L2 vectors containing these adducts and a control were constructed and replicated in Escherichia coli. A remarkable difference in the induced CpG deletion frequency between these adducts was noted. In repair-competent cells the 1-NP adduct induced 1.7% CpG deletions without SOS, whereas the 1,6- and 1,8-DNP adducts induced 6.8 and 10.0% two-base deletions, respectively. With SOS, CpG deletions increased up to 1.9, 11.1, and 15.1% by 1-NP, 1,6-, and 1,8-DNP adducts, respectively. This result unequivocally established that DNP adducts are more mutagenic than the 1-NP adduct in the repetitive CpG sequence. In each case the mutation frequency was significantly increased in a mutS strain, which is impaired in methyl-directed mismatch repair, and a dnaQ strain, which carries a defect in proofreading activity of the DNA polymerase III. Modeling studies showed that the nitro group on the pyrene ring at the 8-position can provide additional stabilization to the two-nucleotide extrahelical loop in the promutagenic slipped frameshift intermediate through its added hydrogen-bonding capability. This could account for the increase in CpG deletions in the M13 vector with the nitro-containing adducts compared with the Gua-C8-AP adduct itself.
authors.affiliation: Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269, USA.
authors.forename: Pablo
authors.initials: P
authors.lastname: Hilario
authors.forename: Shixiang
authors.initials: S
authors.lastname: Yan
authors.forename: Brian E
authors.initials: BE
authors.lastname: Hingerty
authors.forename: Suse
authors.initials: S
authors.lastname: Broyde
authors.forename: Ashis K
authors.initials: AK
authors.lastname: Basu
mesh_terms: D018899:CpG Islands / Q000235:genetics*; D018736:DNA Adducts / Q000737:chemistry / Q000235:genetics*; D004260:DNA Repair; D004277:DNA, Single-Stranded / Q000737:chemistry / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics; D005822:Genetic Vectors; D006860:Hydrogen Bonding; D008958:Models, Molecular; D015394:Molecular Structure; D016296:Mutagenesis; D009153:Mutagens / Q000737:chemistry*; D009690:Nucleic Acid Conformation; D010641:Phenotype; D011721:Pyrenes / Q000737:chemistry*
chemical_list: D018736:DNA Adducts; D004277:DNA, Single-Stranded; D009153:Mutagens; D011721:Pyrenes; C033310:1,8-dinitropyrene; C033309:1,6-dinitropyrene; C032668:1-nitropyrene