ABSTRACT The transcription factor IIIA (TFIIIA) plays a central role in the synthesis of 5S ribosomal RNA (rRNA) by RNA polymerase III. It is the first eukaryotic gene-specific factor to be isolated and has been very extensively studied in many laboratories for over two decades. TFIIIA is unique in that it binds to both 5S rRNA genes and to 5S RNA, the gene product. The TFIIA gene is developmentally regulated. The factor is a zinc metalloprotein and contains nine tandemly repeated zinc finger motifs. The protein in its native form is a single polypeptide encoded by a single copy gene and is composed of two structural domains. The N-terminal domain is required for sequence specific recognition of DNA and RNA but does not support the transcription. Its C-terminal domain, on the other hand, does not bind the DNA but carries out the transactivation processes. TFIIIA promotes the formation of a stable transcription complex and is involved in the assembly of active chromatin template. The factor plays a major role in the differential expression of two kinds of 5S rRNA genes in Xenopus and has become a model protein to study the mechanisms of DNA/RNA/protein interactions.
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