[Differential reaction of polytene chromosome segments to high doses of actinomycin D]

The inhibition of RNA synthesis by a high dose of actinomycin D (10 and 50 mg/ml) in Chironomus thummi polytene chromosomes was followed by drastic changes in the ultrastructure of puffs and some bands: 1) in some puffs, the number of puff granules was sharply reduced, and such puffs were collapsed, 2) in other puffs, the number of puff particles did not decrease but puff granules were substituted by thin fibrils. These puffs remained in an decondensed state and even increased their size, 3) in some puffs, large droplets or thick fibrils appeared never observed in these puffs in normal conditions, 4) some bands are decondensed like the puffs, 5) band segregation (particularly in centromere regions) into two substances with low and high electron densities has been described for the first time. A low electron dense substance was Bernhard--positive and contained RNA. The phenomenon of band segregation has been compared with nucleolar segregation. The results are discussed in relation to the ultrastructural organization of the transcription complex and puff products in polytene chromosomes.