Stable transcription complexes of Xenopus 5S RNA genes: a means to maintain the differentiated state

Cloned 5S RNA genes added to Xenopus oocyte nuclear extract assemble into stable active transcription complexes that persist for many rounds of 5S RNA synthesis. This stability of the complex has been demonstrated by its resistance to dilution and to competitor DNA. A stable complex is formed within minutes and lasts for at least 40 rounds of transcription per template over several hours. Stable, transcriptionally inactive complexes can be formed by incubation of cloned 5S RNA genes in an oocyte nuclear extract depleted of a 5S-specific transcription factor and supplemented with histones. The stable, transcriptionally active and inactive states of 5S RNA gene complexes that can be formed in vitro are analogous to the states of the somatic and oocyte 5S RNA genes as they exist in somatic cell chromatin. Oocyte 5S RNa genes remain repressed in chromatin isolated from somatic cells, but can be activated by washing chromatin with high salt. Maintenance of the differentiated state of cell requires that selected genes remain stably active while others are stably repressed for long periods of time. We propose that stable transcription complexes may play an important role in the maintenance of the differentiated state in eucaryotic cells.