RNA-polymerase specificity of transcription of Arabidopsis U snRNA genes determined by promoter element spacing.

Although most eukaryotic genes known to be transcribed by RNA polymerase III have intragenic promoter elements, some are similar to genes transcribed by RNA polymerase II in that they have upstream promoters (reviewed in refs 1-4). Transcription of the vertebrate U6 and 7SK RNA genes by RNA polymerase III depends exclusively upon upstream signals, some of which are indistinguishable from the elements essential for polymerase II-specific genes. In the plant Arabidopsis thaliana the promoter elements for the U6 and U2 small nuclear RNA genes, transcribed by RNA polymerases III and II respectively, are identical, comprising a -30 TATA box and an upstream element specific for small nuclear RNA genes. The distance between these elements differs, however. Here we report evidence that this separation is crucial in determining whether the genes are transcribed by polymerase II or III.