FUNCTIONAL-ANALYSIS OF ARABIDOPSIS-THALIANA RIBOSOMAL-RNA GENE AND SPACER PROMOTERS IN-VIVO AND BY TRANSIENT EXPRESSION

In eukaryotes, RNA polymerase I transcription is controlled by DNA ele ments located within the spacers that separate the tandemly arranged r RNA genes. Unlike rRNA coding sequences, the intergenic spacers evolve rapidly and have little sequence similarity even among closely relate d species. Nonetheless, the arrangement of functional elements, such a s spacer promoters and enhancers, is thought to be highly conserved. H ere, we identify spacer promoters in the plant Arabidopsis thaliana, t hereby demonstrating their existence in both the plant and animal king doms. We also use an Arabidopsis transient expression system to perfor m transcriptional analysis of the ribosomal gene promoter. Spacer prom oters share sequence similarity with the gene promoter from -91 to +22 relative to the transcription start site, +1. Deletion analysis shows that sequences required for RNA polymerase I transcription reside wit hin these boundaries. Spacer sequences upstream of the gene promoter h ave only a small positive effect on transcription in transfected proto plasts but can increase transcription from a Xenopus ribosomal gene pr omoter in injected frog oocytes. This trans-kingdom enhancer effect fu rther suggests that the functional elements within eukaryotic ribosoma l genes are highly conserved.