[Fluorescence microscopic and autoradiographic analyses of the differential reaction of various regions of polytene chromosomes in Chironomus to prolonged exposure to 7-amino-actinomycin D and 3H-actinomycin D].

Dynamics of binding of a fluorescent analogue of actinomycin D -- 7-amino-actinomycin D -- and 3H-actinomycin D with polytene chromosomes of Ch. thummi was studied. Biological effects of AMD, 7-amino-AMD, and 3H-AMD on polytene chromosomes were found to be similar. These ligands provoke the reduction of the nucleolus and Balbiani rings and the appearance of giant pseudo-puffs in heterochromatic centromere regions of polytene chromosomes. There was no intermediate binding of 7-amino-AMD to DNA in vivo both after a longterm treatment of larvae with fluorochrome and in chase experiments. It was found that a loosening of chromatin in centromere regions accompanied by a weakening of its fluorescence took place in the formation of pseudo-puffs. Possible mechanisms of pseudo-puff formation under the influence of AMD and 7-amino-AMD are discussed. Essential factors may be peculiarities of DNA nucleotide composition in centromere regions, DNA packing, alteration of physico-chemical properties of DNA in the complex with AMD (despiralizations and elongation), and an inhibition of RNA synthesis necessary for the maintenance of normal structure of polytene chromosomes.