Cytofluorometric studies on conformation of nucleic acids in situ. I. Restriction of acridine orange binding by chromatin proteins.

Binding of the fluorochrome acridine orange (AO) to nucleic acids in situ is studied by automated cytofluorometry in two differentiating cell systems: Friend virus-transformed murine erythroleukemia induced to differentiate by dimethyl sulfoxide, and phytohemagglutinin-stimulated human lymphocytes. The specificity of the stain for deoxyribonucleic acid is discussed on the basis of data obtained by cell treatment with nucleases. Evidence is presented that in the case of Friend leukemia cells, but not phytohemagglutinin-stimulated lymphocytes, a significant change in the number of AO-intercalating sites in DNA occurrs during differentiation. These results suggest that changes in nuclear chromatin occurring during cell differentiation may be correlated, in some but not all systems, with changes in accessibility of DNA in situ to intercalating dyes. The role of divalent cations, especially Mg2+, in the conformation of nuclear chromatin and in modulation of the accessibility of nucleic acids to AO is discussed. The method provides a tool for the study of nucleic acid-protein interaction in situ, and in some cell systems it may be applicable as a marker for recognition of cell transformation, differentiation or neoplasia.