FLUORESCENCE OF COMPLEXES OF QUINACRINE MUSTARD WITH DNA. I. INFLUENCE OF THE DNA BASE COMPOSITION ON THE DECAY TIME IN BACTERIA

The fluorescence of several bacterial DNAs stained with quinacrine mustard have been investigated using a laser microfluorometer with a spatial resolution of - 0.3 μm and a temporal resolution of ?0.3 ns connected to a digital signal averager. Experiments performed on Micrococcus lysodeikticus samples show that both cytological preparations and the corresponding purified DNAs give coincident fluorescence curves, thus indicating that the fluorescence observed in the former case is to be attributed to the bacterial DNA only. Experiments thereafter performed on smears of several bacteria with known AT percentages show that each fluorescence decay curve, after a fast transient, can be fitted by an exponential decay law with a single time constant. This time constant has been found to depend linearly on the square of the AT percentage. We explain this result on the basis of an energy transfer mechanism between dye molecules intercalating AT:AT sequences (donors) and dye molecules bound to either GC:GC or GC:AT sequences (acceptors). The agreement with the experimental data requires that all the bacteria considered present a common value for both the number of base pairs contained in a Förster sphere and for the maximum saturation of the strong binding process.