The kinetics of DNA methylation in cultures of a mouse adrenal cell line

Abstract

Abstract Direct measurements of the methylation of newly‐synthesized DNA were made in cultures of a clonal mouse adrenal cortex cell line, Y129OS3, by (1) following the incorporation of radioactivity from methionine‐(methyl)‐C 14 into a segment of DNA which had been density‐labeled with bromouracil and (2) labeling DNA cytosine with C 14 ‐deoxycytidine and then following the appearance of radioactivity in DNA 5‐methylcytosine. The results establish that during exponential growth the DNA of this cell line is methylated entirely within a few minutes of its synthesis. Using the second technique described above accurate, sensitive measurements of DNA methylation levels can be made by comparing radioactivity in 5‐methylcytosine to radioactivity in cytosine plus 5‐methylcytosine. In this cell line 5‐methylcytosine accounts for 4.3 ± 0.2% of the DNA cytosine. Some apparent contradictions between these results and those of other workers are discussed.