Further studies on the mammalian cell nucleolus

A new nucleolar fraction (P 0) that is observed following drug treatment (actinomycin D, proflavine, camptothecin, and adenosine) of tissue culture cells has been identified and its properties are described. In addition, other previously described structural entities that appear as a result of drug treatment have further been analyzed and discussed. All particulate fractions contain fibrils that on the basis of their digestion behavior and structural relationships are suspected to be chromatin in nature. They appear to form a part of a chromatin system that seems to include also the fibrils of fibrillar centers and the greater part of the condensed intra- and perinucleolar chromatin. The function of this chromatin system remains unknown. It is speculated that it might be linked to the processing of nucleolar granules. Evidence which suggests that microspherules represent a condensed form of chromatin has been presented. The appearance of microspherules usually coincides with the inhibition of rRNA synthesis; therefore the question of whether they might contain inactivated ribosomal cistrons has been raised.