SUMMARY: A study has been made of the levels of 6-methylaminopurine and 5-methyl-cytosine in the DNA of during thymine deprivation. While DNA synthesis was inhibited by thymine deprivation, DNA methylation continued. Base analysis indicated that this aberrant methylation involved an increase solely in the amount of 5-methylcytosine. These aberrant 5-methylcytosine residues were removed from the DNA during continued growth of bacteria in medium lacking thymine. In contrast, 5-methylcytosine residues synthesized during normal growth were relatively unaffected by thymine deprivation. The results are interpreted to indicate that the extensive DNA damage which occurs during thymine deprivation is due in part to exonuclease digestion of regions of DNA containing aberrant 5-methylcytosine residues.


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