RNA polymerase III transcription in synthetic nuclei assembled in vitro from defined DNA templates

Although much is known of the basic control of transcription, little is understood of the way in which the structural organization of the nucleus affects transcription. Synthetic nuclei, assembled de novo in extracts of Xenopus eggs, would be predicted to have a large potential for approaching the role of nuclear structure in RNA biogenesis. Synthetic nuclei provide a system in which the genetic content of the nuclei, as well as the structural and enzymatic proteins within the nuclei, can be manipulated. In this study, we have begun to examine transcription in such nuclei by using the most simple of templates, RNA polymerase III (pol III)-transcribed genes. DNA encoding tRNA or 5S genes was added to an assembly extract, and nuclei were formed entirely from the pol III templates. Conditions which allowed nuclear assembly and pol III transcription to take place efficiently and simultaneously in the assembly extract were found. To examine whether pol III transcription could initiate within synthetic nuclei, or instead was inhibited in nuclei and initiated only on rare unincorporated templates, we identified transcriptional inhibitors that were excluded from nuclei. We found that these inhibitors, heparin and dextran sulfate, blocked pol III transcription in the absence of assembly but did not do so following nuclear assembly. At the concentrations used, the inhibitors had no deleterious effect on nuclear structure itself or on nuclear import. We conclude that pol III transcription is active in synthetic nuclei, and this conclusion is further strengthened by the finding that pol III transcripts could be coisolated with synthetic nuclei. The rapid and direct transcriptional analysis possible with pol III templates, coupled with the simple experimental criteria developed in this study for distinguishing between nuclear and non-nuclear transcription, should now allow a molecular analysis of the effect of nuclear structure on transcriptional and posttranscriptional control.