Specific strand loss in N-2-acetylaminofluorene-modified DNA

Area of Science:

• Molecular Biology
• Genetics
• Chemical Carcinogenesis

Background:

• N-2-Acetylaminofluorene (AAF) is a chemical carcinogen forming premutagenic lesions on guanine residues.
• These AAF adducts are known to cause frameshift mutations in vivo, potentially by blocking DNA replication forks.
• The SOS response in Escherichia coli is implicated in the mutagenic processing of AAF adducts.

Purpose of the Study:

• To investigate the in vivo consequences of AAF adducts on DNA strand segregation and mutagenicity.
• To determine how AAF adducts in one or both DNA strands of a plasmid affect bacterial transformation and mutation.
• To elucidate the mechanism by which AAF adducts lead to mutations, focusing on strand loss versus direct mutagenesis.

Main Methods:

• Construction of plasmids with site-specific AAF adducts in one or both DNA strands.
• Genetic labeling of DNA strands using a single base-pair mismatch in the tetracycline-resistance gene.
• Transformation of bacterial cells with these modified plasmids and analysis of resulting clones.
• Sequencing of mutations in selected bacterial mutants to identify their origin.

Main Results:

• Plasmids with AAF adducts on only one strand consistently led to the loss of that damaged strand in transformants (90%).
• Plasmids with AAF adducts on both strands resulted in pure clones carrying either the wild-type or mutant allele.
• Forward mutation frequency was over ten times lower when AAF adducts were on a single strand compared to both strands.

Conclusions:

• Single-strand AAF adducts trigger specific loss of the damaged DNA strand, rather than direct mutagenesis.
• This strand loss mechanism significantly reduces the overall mutagenic efficiency of AAF adducts.
• Observed background mutations suggest the presence of other cryptic DNA lesions processed by the SOS response.