The crystal structure of d(GGATGGGAG): an essential part of the binding site for transcription factor IIIA

Most genes in higher organisms are activated by the binding of proteins called transcription factors. One such protein, transcription factor IIIA (TFIIIA) from the frog, activates the gene for 5S RNA by binding to the region of the gene between nucleotides 45 and 97. This binding site has been defined by a variety of biochemical studies, including base-deletion experiments and DNase I footprinting. The protein also binds to the gene product: in immature frogs it is stored as a complex with 5S RNA. From the observation that TFIIIA can bind to either double-helical DNA or RNA, and from their own measurements, Rhodes and Klug have proposed that the DNA-binding site for TFIIIA has an RNA-like structure. Here we present the crystal structure analysis of a part of the DNA-binding site (nucleotides 81-89 of the gene) which forms a particularly strong interaction with the protein, and show that it has a conformation similar to the A' form of double-helical RNA.