A factor with Sp1 DNA-binding specificity stimulates Xenopus U6 snRNA in vivo transcription by RNA polymerase III

Abstract

We have previously shown that transcription of the Xenopus U6 snRNA gene by RNA polymerase III is stimulated in injected Xenopus oocytes by an activator element termed the DSE, which contains an octamer sequence. Data presented here reveal that the DSE contains, in addition, a GC-rich sequence capable of binding Spl. Both elements are required to obtain wild-type levels of U6 transcription in vivo. The Xenopus U6 DSE exhibits optimal activation properties only when positioned at its normal location upstream from the start site. The U6 Spl motif binds the mammalian Spl transcriptional activator independently of the Oct-1 protein in vitro. Those mutations that lead to a reduced transcription level in vivo abolish the binding of Spl in vitro. Thus, transcriptional stimulation through the Xenopus U6 Spl motif is likely to be mediated by a protein with DNA-binding specificity identical to mammalian Spl. These findings support the notion that RNA polymerase II and III transcription complexes share transactivators.