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M(1)dG (3-(2'-deoxy-beta-d-erythro-pentofuranosyl)pyrimido[1,2-a]purin-10(3H)-one) lesions are mutagenic in bacterial and mammalian cells, leading to base substitutions (mostly M(1)dG to dT and M(1)dG to dA) and frameshift mutations. M(1)dG is produced endogenously through the reaction of peroxidation products, base propenal or malondialdehyde, with deoxyguanosine residues in DNA. The mutagenicity of M(1)dG in Escherichia coli is dependent on the SOS response, specifically the umuC and umuD gene products, suggesting that mutagenic lesion bypass occurs by the action of translesion DNA polymerases, like DNA polymerase V. Bypass of DNA lesions by translesion DNA polymerases is conserved in bacteria, yeast, and mammalian cells. The ability of recombinant human DNA polymerase eta to synthesize DNA across from M(1)dG was studied. M(1)dG partially blocked DNA synthesis by polymerase eta. Using steady-state kinetics, we found that insertion of dCTP was the least favored insertion product opposite the M(1)dG lesion (800-fold less efficient than opposite dG). Extension from M(1)dG.dC was equally as efficient as from control primer-templates (dG.dC). dATP insertion opposite M(1)dG was the most favored insertion product (8-fold less efficient than opposite dG), but extension from M(1)dG.dA was 20-fold less efficient than dG.dC. The sequences of full-length human DNA polymerase eta bypass products of M(1)dG were determined by LC-ESI/MS/MS. Bypass products contained incorporation of dA (52%) or dC (16%) opposite M(1)dG or -1 frameshifts at the lesion site (31%). Human DNA polymerase eta bypass may lead to M(1)dG to dT and frameshift but likely not M(1)dG to dA mutations during DNA replication.
A highly efficient, non-labor-intensive method for cloning DNA fragments produced by PCR amplification was used to carry out a rapid survey of potential point mutations in integrin alpha 6 cDNA from 17 different cell-type sources. The method includes glass powder purification of the PCR reaction mixture, followed by simultaneous treatment with T4 polynucleotide kinase and DNA polymerase I, and another glass powder purification. Sequences from multiple subclones of each cell type were readily generated, aligned and checked for mismatches. Several commonly used alternative procedures were compared for cloning efficiency and size-fidelity of inserted DNA fragments.