In situ evaluation of the functional state of chromatin by means of Quinacrine Mustard staining and time-resolved fluorescence microscopy.

A pulsed laser microfluorometer with high spatial and temporal resolution was employed to study the functional state of chromatin, using Quinacrine Mustard as the fluorescent probe. Corresponding segments of polytene chromosomes of embryo suspensor cells of Phaseolus coccineus and parenchymal cells of Helianthus tuberosus, in phases not involving DNA synthesis, were selected as models. The fluorescence decay time turned out to be a discriminating parameter for the chromatin fractions of differing functional engagement. The results were interpreted on the basis of a different accessibility of the DNA to the intercalating agent, as a result of the different structural situation of the chromatin. This fact determines modifications of the energy transfer rates between dye molecules in different quantum efficiency conditions, which results in variations of fluorescence decay times.