STAF, A NOVEL ZINC-FINGER PROTEIN THAT ACTIVATES THE RNA-POLYMERASE-III PROMOTER OF THE SELENOCYSTEINE TRANSFER-RNA GENE

The selenocysteine tRNA gene (tRNA(Sec)) is atypical. Though transcrib ed by RNA polymerase III like all other tRNA genes, its basal promoter elements are distinct and reside essentially upstream of the coding r egion. In addition, transcription from the basal promoter is activated by a 15 bp activator element. In this report we describe the cloning and functional characterization of Staf (selenocysteine tRNA gene tran scription activating factor), a novel Xenopus laevis transcription fac tor which binds to the tRNA(Sec) activator element and mediates its ac tivation properties. The 600 amino acid Staf protein contains seven zi nc fingers and a separate acidic activation domain. Seven highly conse rved regions were detected between Staf and human ZNF76, a protein of unknown function, thereby aiding in predicting the locations of the fu nctional domains of Staf. With the use of a novel expression assay in X. laevis oocytes we succeeded in demonstrating that Staf can activate the RNA polymerase III promoter of the tRNA(Sec) gene. This constitut es the first demonstration of the capacity of a cloned factor to activ ate RNA polymerase III transcription ia vivo.