Binding of histones to Xenopus laevis ribosomal genes with different levels of expression.

The association of ribosomal RNA genes with histones as a function of their expression has been studied in Xenopus laevis erythrocytes, where the genes are silent, and tadpoles at stage 40, where these genes are actively transcribed. Isolated nuclei were either treated with formaldehyde or irradiated with an ultraviolet laser to cross-link proteins to DNA. The covalently linked protein-DNA complexes were purified by centrifugation through CsCl and immunoprecipitated with antibodies against H1, H2A and H4.