Changing the RNA polymerase specificity of U snRNA gene promoters.

The promoter of a Xenopus tropicalis U6 gene can be transcribed by both RNA polymerases II and III. Two distinct elements, a TATA-like sequence and the region of transcription initiation, are only required for transcription by RNA polymerase III, while further common elements are required for transcription by both polymerases. Based on the unusually stringent requirement for a purine at the normal position of polymerase III transcription initiation and on the properties of mutants in this region, we suggest that RNA polymerase III itself may recognize the site of transcription initiation and thus be directly involved in efficient promoter selection. We have used the information obtained on U6 promoter structure to manufacture a U6 promoter that is RNA polymerase II-specific and to change the Xenopus U2 gene promoter specificity from RNA polymerase II to RNA polymerase III.