Résumé
Chromatin structure is known to influence class III gene expression in vitro. We describe the active transcription of Xenopus class III genes following replication and assembly into chromatin by using Xenopus egg extracts. Changes in the structure of this active chromatin dependent on the presence of exogenous Mg
2+
ATP or on the addition of a mixture of histones H2A and H2B are shown to lead to the selective repression of Xenopus 5S RNA genes. Preexisting transcription complexes on 5S DNA are disrupted following the reorganization of a "disordered" histone-DNA complex into a structure consisting of physiologically spaced nucleosomes. Thus, we demonstrate that chromatin structural transitions can have dominant and specific effects on transcription.