Effect of topological constraint on transcription of ribosomal DNA in Xenopus oocytes. Comparison of plasmid and endogenous genes.

We have analyzed the effects of topological constraint on the transcription of both injected ribosomal DNA plasmids and the endogenous ribosomal genes in Xenopus oocytes. Efficient transcription of injected ribosomal gene plasmids requires a covalently closed circular template. Once transcription is initiated on injected plasmids there is a continuous requirement for topological constraint, since subsequent cutting with a restriction endonuclease abolishes transcription. In contrast, both initiation and elongation of transcription on endogenous ribosomal genes are maintained after cutting with restriction endonucleases.