DNA methylationin normal and tumour virus-transformed cells in tissue culture I. The level of DNA methylation in BHK21 cells and in BHK21 cells transformed by polyoma virus (PyY cells)

Abstract

The proportion of the cytosine moieties in DNA which are methylated to 5-methylcytosine at the macromolecular level has been determined in BHK21 cells and compared with the proportion of this minor base in the DNA of polyoma virustransformed BHK21 cells under identical conditions. Almost twice as much 5-methylcytosine was detected in the DNA of the PyY cells as was present in the BHK21 cells, both quantities being calculated relative to the total cytosine in the DNA of the relevant cell line. The possibility that 5-methylcytosine could also arise from the amination of thymine within the DNA was investigated. No evidence for the operation of this pathway was detected within the experimental limits, which would have detected 0.2 % amination of the thymine of either cell line. When the cells were grown at lower densities the proportion of 5-methylcytosine in the DNA of both cell lines was increased, but the difference between the levels in each cell line was maintained. The possibility that the level of 5-methylcytosine in DNA in mammalian cells could alter with their rate of DNA synthesis, or their degree of contact inhibition, was investigated by synchronising the cells with deoxyadenosine. Different levels of methylation were observed in both cell lines at different rates of DNA synthesis.