Organization, replication and modification of the human genome: differential methylation of two classes of HeLa nuclear DNA separated on Ag+--Cs2SO4 gradients.

HeLa nuclear DNA sediments as a single peak, in neutral CsCl, while it is separated in a heavier and a lighter components, in alkaline Ag+--Cs2SO4. The heavy fraction, on the average, represents about 20% of the total DNA. CsCl analytical ultracentrifugation shows that heavy DNA bands at 1.715 g/cm3 and contains 53% GC (10% of the total GC), whereas light DNA bands at 1.703 g/cm3 and contains 40% GC (32% of the total GC). Coherently, Tm values in 0.1 x SSC are 82.5 degrees C, for heavy DNA, and 72.5 degrees C, for light DNA. After treatment with [3H-methyl-S-adenosyl-L-methionine in isolated nuclei, the concentration of labelled 5-methylcytosine was found to be highest in the more dense regions of the heavy peak and in the less dense regions of the light peak. Exposure to ultrasound modifies the quantitative relationship of the two peaks and improves the separation of supermethylated AT- and GC-rich DNAs. Four possible triplets as sites for DNA-methylase recognition are discussed.