Chromatin and histones in mealy bug cell explants: activation and decondensation of facultative heterochromatin by a synthetic polyanion

In spermatogonial cells of the mealy bug, Planococcus citri, at interphase the five maternal chromosomes appear as diffuse euchromatin and the five paternal chromosomes are heterochromatic, genetically inactive, and incorporate tritiated uridine into RNA at a diminished rate. Testes squashes were treated with 2–10 mg/ml of the polyanion, polystyrene sulfonate (PSS). The gonial cell nuclei decondensed and after 15 minutes they became uniformly granular and similar in appearance to wholly euchromatic nuclei. When testis expiants were incubated with PSS (2–10 mg/ml) for from 15 to 120 minutes, all stages of deheterochromatization were recovered. The Feulgen reaction revealed that the uniform granules contained DNA; methyl-green-thionin staining indicated that the nucleolus contained RNA. When tritiated uridine was added after 15 minutes of PSS and then incubation continued, autoradiography revealed incorporation into euchromatin and decondensing heterochromatin. Incorporation of uridine increased with dosage of PSS up to 4 mg/ml. PSS (20 mg/ml) was toxic to the cells: They incorporated no uridine and were badly damaged. RNAase treated controls were also devoid of label.—PSS treated cells showed a negative alkaline-fast-green reaction for histone. In vitro a complex was formed between calf thymus histone and PSS which was soluble only above pH 8.5, but not separable on a Dowex acetate ion exchange column. These findings suggest that, probably by disrupting the structure of the DNA-histone complex, polystyrene sulfonate brings about structural decondensation of heterochromatin and enables it (and euchromatin) to incorporate tritiated uridine into RNA at an increased rate.