Transcription factor IIIA induced bending of the Xenopus somatic 5S gene promoter

Abstract

TRANSCRIPTION factor IIIA (TFIIIA), the canonical zinc-finger protein, is a protein of relative molecular mass 39,000 (39K) that is required for transcription of 5S-ribosomal subunit genes in Xenopus¹. It binds in a sequence-specific manner to the internal control region of the 5S gene (see Fig. 1) and facilitates transcrip-tion of the gene by RNA polymerase III^2,3. It also binds to the 5S gene product to form a 7S ribonucleoprotein particle. In oocytes the 7S particle acts as a storage form of the RNA to be utilized later in development^4,5. TFIIIA binds to DNA through its 30 K N-terminal domain, which contains nine zinc-fingers⁶. TFIIIA was the first protein described to have this type of DNA binding motif, but numerous other proteins have now been shown to have zinc-finger domains⁷. A structure for a single zinc-finger from the yeast protein ADR1, was recently proposed based on two-dimensional NMR data (ref. 8), and a similar structure was proposed based on comparison with crystal structures of other metalloproteins⁹. Although models for the interaction of TFIIIA with the 5S-ribo-somal gene DNA have been proposed, based on nuclease digestion and methylation interference data¹⁰, little precise structural infor-mation is available for TFIIIA and the physical basis for the interaction of zinc-fingers with DNA is not understood. Using both circular permutation and circularization assays we provide convinc-ing biochemical evidence that TFIIIA bends the DNA at the internal promoter of the 5S gene.