Decrease in the number of DNA topological turns during Friend erythroleukemia differentiation

Murine erythroleukemia cells were induced to undergo erythroid differentiation by growing in presence of dimethylsulfoxide, butyric acid or actinomycin D. topological linking numbers in closed loops of nuclear DNA were measured by means of centrifugation of nucleoids containing superhelical DNA in sucrose gradients containing varying concentrations of ethidium bromide. All cells were grown to G1 stage of the cell cycle. It was found that the mean density of the DNA topological linking number decreases from 0.076 turns per 10 nucleotide pairs in non-differentiated cells to 0.062 turns in the cells induced to differentiate. This decrease in topological linking number of DNA loops is quite sufficient for the change in the DNA double helix secondary structure which in turn may be responsible for coordinate switch in transcription of genes which control cellular differentiation (Luchnik, 1980a, b).