Abstract
The genome of Xenopus laevis contains two large families of class III genes (oocyte 5S RNA and satellite I DNA) that are repressed in somatic cells. Both gene families are actively transcribed in a soluble extract of X.laevis oocyte nuclei, using chromatin deficient in histone H1 as a template. The addition of histone H1, to this transcriptionally active chromatin, results in the dominant and selective repression of oocyte 5S RNA genes and satellite I DNA. Somatic 5S RNA genes remain active following histone H1 addition. Changes in chromatin structure could have a dominant role in the regulation of class III gene expression during Xenopus embryogenesis.