Abstract
Polystyrene sulfonate (PSS) was used to determine the relationship between histones and the transcriptional availability of facultative heterochromatin and euchromatin in mealy bug cell explants. PSS decondenses facultative heterochromatin, activates RNA synthesis in goniai cells and removes histones in a specific sequence. The removal of histone makes template available for the binding of actinomycin D (AMD). In metabolically active goniai cells, PSS (2 mg/ml) treated heterochromatin incorporated 11.3 times more 3H-AMD than controls, while PSS treated euchromatin incorporated only 1.1 times more 3H-AMD than controls. The ratio of mean grain density over euchromatin to heterochromatin decreased from a control value of 7.65 to approximate unity at PSS concentrations greater than 2 mg/ml. The same disproportionate increases in label over heterochromatin and euchromatin were seen when testis explants were incubated in unlabelled AMD before being transferred to PSS (2–5 mg/ml) and 3H-AMD, indicating that new DNA sites were being made available as histones were removed. These data suggest that the degree of condensation, and, hence, of template availability of interphase chromatin is related to the firmess of the bond between DNA and histone.
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