Flow cytometric evaluation of DNA stainability with propidium iodide after histone H1 extraction

A flow cytometric evaluation of the effect of the histone H1 extraction on DNA stainability with propidium iodide was performed on isolated HeLa nuclei. Selective removal of the lysine-rich protein was attained by using two established techniques involving treatment with 0.7 M NaCl or low pH. DNA stainability was monitored at different dye/DNA-P ratios, varying from low to high saturating concentrations. Depletion of the histone H1from nuclei results in the transition from low to high affinity of a portion of binding sites, as shown by 1) the increase in fluorescence intensity after staining with the dye at low saturating concentrations and 2) the higher value of the fluorescence intensity ratio (FI5/FI50) exhibited by H1-depleted nuclei stained with a low (5 micrograms/ml) vs. a high (50 micrograms/ml) concentration, as compared with control samples.