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Extrachromosomal Probes for Mutagenesis by Carcinogens: Studies on the Mutagenic Activity of O6-Methylguanine Built into a Unique Site in a Viral Genome

John M. Essigmann, Kerry W. Fowler, Calvert L. Green and Edward L. Loechler
Environmental Health Perspectives
Vol. 62 (Oct., 1985), pp. 171-176
DOI: 10.2307/3430109
Stable URL: http://www.jstor.org/stable/3430109
Page Count: 6
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Extrachromosomal Probes for Mutagenesis by Carcinogens: Studies on the Mutagenic Activity of  O6-Methylguanine Built into a Unique Site in a Viral Genome
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Abstract

This work examines the mutagenic activity of O6-methylguanine( O6 MeGua), a DNA adduct formed by certain carcinogenic alkylating agents. A tetranucleotide, 5′- OH T p m6 G p C p A-3′, was synthesized and ligated into a four-base gap in the unique Pst I site of the duplex genome of the E. coli virus, M13mp8. The double-stranded ligation product was converted to single-stranded form and used to transform E. coli to produce progeny phage. The mutation frequency of O6 MeGua was defined as the percentage of progeny phage with mutations in their Pst I site, and this value was determined to be 0.4%. To determine the impact of DNA repair on mutagenesis, cellular levels of O6 MeGua- DNA methyltransferase (an O6 MeGua-repair protein) were depleted by treatment of host cells for virus replication with N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) prior to viral DNA uptake. In these host cells, the mutation frequency due to O6 MeGua increased markedly with increasing MNNG dose (the highest mutation frequency observed was 20%). DNA sequence analysis of mutant genomes revealed that in both MNNG treated and untreated cells, O6 MeGua induced exclusively G to A transitions.

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