cis-acting enhancement of RNA polymerase III gene expression in vitro.

The Xenopus laevis S-150 cell-free extract catalyzes in vitro transcription of several RNA polymerase III genes. Among these are the Xenopus 5S RNA gene (somatic type) and the Xenopus methionine tRNA gene. In this report we present an analysis of the transcriptional activity of these two genes either in trans-competition experiments or when the genes are co-localized in the same circular plasmid. In the "cis" arrangement, elevated levels of 5S and tRNA gene expression are observed, which are dependent on the relative orientation of the two genes (convergent or in tandem) and the distance between them. The results of these analyses reveal important parameters affecting the expression of juxtaposed genes.