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leukemia cells from exponentially growing or differentiated (
DMSO-induced) cultures were permeabilized and their
DNA was stained with 4'6-diamidino-2-phenylindole (
DAPI),
Hoechst 33342,
acridine orange,
ethidium bromide,
propidium iodide,
quinacrine,
7-amino-actinomycin D,
mithramycin, or
chromomycin A3. Accessibility of
DNA to each of the above
fluorochromes was compared in differentiated and nondifferentiated cells before and after
nuclear proteins, mostly
histones, were extracted with 0.1N HCl. A decrease in the accessibility of
DNA to several
dyes, especially pronounced in the case of some
intercalators, was observed in differentiated cells. After extraction of
nuclear proteins with HCl there was an increase in
DNA accessibility, of varying degree depending on the
fluorochrome and the difference between differentiated and nondifferentiated cells was abolished for most of the intercalating
dyes. The increase was the lowest for
DAPI (45%), the highest for
7-amino-actinomycin D (13-fold), and in general was higher for the intercalating
dyes that unwind
DNA than for
dyes binding externally to the double helix. The results are discussed in terms of the mode of interactions between
DNA and the
fluorochromes and factors associated with
chromatin structure that may affect accessibility of
DNA in situ in exponentially growing and differentiated cells.