Printed in Great Britain Aberrant DNA Methylation under Conditions of Thymine Deprivation in Bacillus subtilis

A study has been made of the levels of 6-methylaminopurine and ymethyl-cytosine in the DNA of B d c i h subtilis 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 depriva-tion is due in part to exonuclease digestion of regions of DNA containing aberrant 5-methylcytosine residues