Template length, sequence context, and 3'-5' exonuclease activity modulate replicative bypass of thymine glycol lesions in vitro.
cis-Thymine glycol, a product of ionizing radiation damage to DNA, has been introduced quantitatively at a single site into oligonucleotide templates. The ability of DNA polymerases to replicate templates containing thymine glycol was studied by a primer extension assay, and three factors that influence replicative bypass of this lesion in vitro have been identified. These factors include template length, sequence context, and 3'-5' exonuclease activity. Synthesis by the large fragment of DNA polymerase I (Klenow fragment) terminates quantitatively opposite thymine glycol when the template strand extends only two nucleotides beyond the lesion. Significant bypass is observed when the length of the template beyond the lesion is increased to six nucleotides. On the longer templates, the frequency of bypass of the Klenow fragment depends upon the identity of the base immediately 5' to thymine glycol. The extent of bypass is greatest with cytosine and least with adenine at this position. Bypass of thymine glycol lesions by DNA polymerase alpha 2 from HeLa cells shows a qualitatively similar dependence upon local sequence context. In contrast, synthesis by T4 DNA polymerase is quantitatively blocked opposite the lesion regardless of template length or DNA sequence context. Synthesis by a mutant Klenow fragment that is deficient in 3'-5' exonuclease activity, or by AMV reverse transcriptase, results in a significant increase in the frequency of bypass. Thus, increased nucleotide turnover at, or beyond, the site of the lesion is likely to contribute significantly to the arrest of synthesis provoked by cis-thymine glycol in vitro.