Xenopus transcription factor IIIA

The Xenopus transcription factor IIIA not only acts as an essential RNA polymerase transcription factor for the expression of the 5S rRNA gene, it also binds to the 5S rRNA to form a 7S ribonucleoprotein particle that stabilizes the RNA until it is required for ribosome assembly and facilitates nuclear export of the 5S rRNA. Indeed, it was originally shown to be the protein component associated with 5S rRNA in the 7S particle in Xenopus oocytes before it was recognized as a transcription factor. How, we may ask, can this protein not only recognize specific DNA sequences in the 5S rRNA gene upstream region, but also recognize different, but equally specific, sequences in 5S rRNA ... 

Aaron Klug discovered the first of the eukaryotic DNA-binding motifs in Xenopus transcription factor IIIA (TFlllA), a protein which binds to the 5S rRNA gene. The resulting complex subsequently binds two other transcription factors and RNA polymerase III, which leads to the initiation of transcription of the 5S rRNA gene. The TFIIIA molecule contains 9 similar 30-residue-long, tandemly repeated modules. Each of these modules contains two invariant Cys residues, two invariant His residues, and several conserved hydrophobic residues (Figure 12.19), and a Zn + ion, which is tetrahedrally coordinated by the invariant Cys and His... 

Westmark CJ, Ghose R, Huber PW. 2002. Phosphorylation of Xenopus transcription factor IIIA by an oocyte protein kinase CK2. Biochem J 362(Pt 2) 375-382. 

Zinc finger a DNA binding motif first recognized in Xenopus transcription factor IIIA (which binds to the internal control sequence of the 5S rRNA gene). This protein contains nine 30-residue sequences, all rather similar and arranged in tandem. Each contains two invariant Cys residues and two invariant His residues, as well as several conserved hydrophobic residues. Each of these structural domains folds about a single Zn ion, and X-ray analysis indicates that the zinc ion is tetrahedrally liganded by the invariant... 

The requirement for zinc in the regulation of gene expression is exemplified by transcription factor IIIA, which has been shown to contain from two (Hanas et al., 1983) to seven to eleven (Miller et al., 1985) zinc ions bound to a 40K protein molecule. Transcription factor IIIA is obtained from immature Xenopus oocytes in a complex with 5 S RNA, and the protein is required for transcription initiation the apoprotein does not bind to the 5 S RNA gene (Hanas et al, 1983). Uncertainties regarding the stoichiometric requirement for zinc persist, given the report by Shang et al. (1989) that transcription factor IIIA, as either the isolated protein or the 5 S RNA complex, contains two firmly bound zinc ions which are required for transcription activation. 

Miller, J. (1985) Repetitive zinc-binding domains in the protein transcription factor IIIA from Xenopus oocytes. EMBO J 4 1609-1614. 

The Xenopus 5S RNA gene-specific transcription factor IIIA (TFIIIA) was the first zinc finger protein characterized one decade ago (Klug and Rhodes 1987 Berg 1990 Rhodes and Klug 1993). This protein specifically binds to... 

Miller J, McLachlan AD, Klug A (1985) Repetitive zinc-binding domains in the protein transcription factor IIIA from Xenopus oocytes. EMBO J 4 1609-1614 Nemer M, Travaglini EC, Rondinello E, D Alonzo J (1984) Developmental regulation, induction and embryonic tissue specificity of sea urchin metallothionein gene expression. Dev Biol 102 471-482... 

---