STRUCTURE, FUNCTION, EVOLUTION OF TRANSCRIPTION FACTOR IIIA

This chapter discusses the structure, function, and evolution of transcription factor (TF) IIIA. The large amount of cysteine in TFIIIA prompted an examination of the possible roles of metals in TFIIIA structure and function because cysteine is a metal chelator and not usually found in such large amounts in intracellular proteins. The zinc present in TFIIIA is loosely bound, as evidenced by complete chelation under very mild conditions. The primary structure of TFIIIA can be modeled into nine zinc binding domains, termed as “zinc finger,” in which a zinc ion is coordinated between two cysteines and two histidines. Finger region is the DNA-binding entity of the protein. TFIIIA has been initially shown to be required for the transcription of 5-S genes in cell extracts. in addition, TFIIIA promotes RNA polymerase-III-dependent transcription of 5-S genes by binding to the internal control region. in trying to understand the role of TFIIIA in the transcription of 5-S RNA genes, it is probably helpful to understand the way factors IIIC and IIIB and RNA polymerase III interact and promote the transcription of both tRNA and 5-S RKA genes. Moreover, the cloning and sequencing of TFIIIA cDNAs from other amphibian species have revealed that TFIIIA is not as highly conserved at the amino-acid level as are other DNA-binding proteins. © 1992 Academic Press, Inc.