Difference in effects of proflavine and actinomycin D on mammalian cell nucleoli.

Fine structural nuclear and nucleolar changes that are inducible in ME-180 tissue culture cells by proflavine and actinomycin D are compared. Treatment with both compounds results in nucleolar segregation, but actinomycin D, in addition, gives rise to a granular fraction, the P2 fraction, which never forms under the action of proflavine. It can be produced, however, by adding actinomycin D to the proflavine medium. There is good evidence that the P2 fraction evolves from the P1 fraction, the regular granular fraction of nucleoli. How actinomycin D brings about the P1 to P2 conversion remains unexplained. Low concentrations of proflavine (1 eg/ml) lead to the formation of fibrillar nodules that, in every respect, are equivalent to the dense fibrillar substance of nucleoli. These nodules show a close association with interchromatin granules. They form in the presence of high doses of puromycin and cycloheximide, but their formation is prevented by actinomycin D in a dose that primarily inhibits ribosomal RNA synthesis. Enzyme digestion results suggest that the nodules consist primarily of pepsin-digestible proteins and fibrils of unknown nature. They are likely to contain also RNA. Similar nodules can also be induced with cordycepin, an adenosine analog (3-deoxyadenosine).