Nucleolar organization in the euglenoid Astasia longa as disclosed by selective staining, actinomycin D treatment, and cold shock

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Abstract
Nucleolar ultrastructure in Astasia longa (Jahn) was examined in normal cells as well as in cells treated with actinomycin D or subjected to low temperature. Selective staining for ribonucleo-proteins (RNP) revealed that, in normal cells, the fibrillar nucleolar zones consisted of a rather homogeneous, moderately dense material within which were observed circular and elongate areas having a noticeably greater density. These denser areas were interpreted as corresponding to cross and oblique sections of a coarse filamentous structure, or nucleolonema, which constitutes the main component of the fibrillar zones. Following staining with Moyne's Feulgen-like technique, the nucleolonemal profiles appeared denser than the remaining regions of the nucleolar mass, thus indicating that DNA is associated with this filamentous structure. This conclusion was supported by the observation that actinomycin D, a substance known to react with DNA, rapidly induced a transformation of the fibrillar nucleolar zones. After 1 h of such treatment and staining for RNP, the nucleolus was observed to be homogeneously dense except for the presence of a number of small, circular, lighter areas presumably representing cross sections of the transformed nucleolonema. Staining for DNA revealed similar masses which were now denser than the remaining portion of the nucleolus. As a result of prolonged treatment with actinomycin D, the Feulgen-positive material often appeared as a coarse network segregated within a crescent-shaped and peripheral region of the nucleolar body.After exposure of cells to low temperature (14.5 °C) for 1 h the granular zones of the nucleolus became looser and the nucleolonemal core appeared less distinct. Nucleolar organization reverted almost to normal when the cold shock treatment was prolonged for a 3-h period.