THE CLONED RNA POLYMERASE-II TRANSCRIPTION FACTOR-IID SELECTS RNA POLYMERASE-III TO TRANSCRIBE THE HUMAN U6 GENE INVITRO

Although the human U2 and U6 snRNA genes are transcribed by different RNA polymerases (i.e., RNA polymerases II and III, respectively), their promoters are very similar in structure. Both contain a proximal sequence element (PSE) and an octamer motif-containing enhancer, and these elements are interchangeable between the two promoters. The RNA polymerase III specificity of the U6 promoter is conferred by a single A/T-rich element located around position-25. Mutation of the A/T-rich region converts the U6 promoter into an RNA polymerase II promoter, whereas insertion of the A/T-rich region into the U2 promoter converts that promoter into an RNA polymerase III promoter. We show that this A/T-rich element can be replaced by a number of TATA boxes derived from mRNA promoters transcribed by RNA polymerase 11 with little effect on RNA polymerase III transcription. Furthermore, the cloned RNA polymerase 11 transcription factor TFIID both binds to the U6 A/T-rich region and directs accurate RNA polymerase Ill transcription in vitro. Mutations in the U6 A/T-rich region that convert the U6 promoter into an RNA polymerase 11 promoter also abolish TFIID binding. Together, these observations suggest that in the human snRNA promoters, unlike in mRNA promoters, binding of TFIID directs the assembly of RNA polymerase Ill transcription complexes, whereas the lack of TFIID binding results in the assembly of RNA polymerase 11 snRNA transcription complexes.