INCORPORATION OF ETHIDIUM-BROMIDE IN THE DROSOPHILA SALIVARY-GLAND APPROACHED BY MICROSPECTROFLUOROMETRY - EVIDENCE FOR THE PRESENCE OF BOTH FREE AND BOUND DYE IN THE NUCLEI OF CELLS IN VIABLE CONDITIONS

The incorporation of 10(-6) M ethidium bromide (EB) was studied in via ble Drosophila melanogaster salivary glands with a spatial resolution reaching a few mu m(3), using a confocal laser microspectrofluorometer designed for spectral analysis. Spectra were recorded with the 514 nm Argon laser line during excitation times of 1 second (20 mu W on the preparation) at 5 min intervals for 30 or 60 min, either at points in determined cell sites or serially throughout the cells. The fluorescen ce intensity time-course indicated that the EB intake was not an all-o r-none process, but rather a graded, sensitive indicator of the functi onal state of the cell. On the micrometer scale, the cytoplasm behaved as an homogeneous substrate with the fluorescence intensity depending on EB intake and intracellular diffusion. In the nucleus? however, lo calized enhancement of the emission intensity was observed. Spectral a nalysis allowed us to characterize the interactions. The mean values o f lambda(max) in the cytoplasm (600 nm), in the nucleus (601 nm) and o utside the glands (602 nm) were less than for free EB in aqueous solut ion (630 nm); values of full width at half maximum were between 92 and 96 nm, which is much lower than the 120 nm observed for free EB. The recorded spectra were analyzed using a linear combination of two spect ral models, namely free and DNA intercalated EB. In the nucleus, the f ree EB model spectra was found to represent up to 10% of the recorded spectra whereas it was near zero in the cytoplasm. The present data su ggest that the intranuclear concentration of free EB (allowing for its lower fluorescence quantum yield) might be at least equal to that of the bound EB.