Replication through an abasic DNA lesion: structural basis for adenine selectivity.

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Abasic sites are frequent DNA lesions with high mutagenic potential, often leading to mutations in human cancers.
• DNA polymerases bypass these lesions, frequently inserting adenine (A-rule), despite the absence of genetic information.

Purpose of the Study:

• To elucidate the structural mechanism by which DNA polymerases incorporate nucleotides opposite abasic sites.
• To investigate the role of specific amino acid residues in directing nucleotide selection during abasic site bypass.

Main Methods:

• X-ray crystallography of DNA polymerase complexes.
• Site-directed mutagenesis to alter active site residues.

Main Results:

• X-ray structures revealed a conserved tyrosine residue in motif B that occupies the space of the absent template base.
• This tyrosine mimics a pyrimidine, promoting preferential incorporation of purines (A-rule) due to optimal geometric fit.
• Mutating tyrosine to tryptophan switched specificity to pyrimidine incorporation.

Conclusions:

• A general amino acid templating mechanism governs the bypass of non-instructive DNA lesions by DNA polymerases.
• This mechanism, involving conserved residues like tyrosine, ensures accurate nucleotide insertion opposite abasic sites, minimizing mutations.