Xenopus transcription factor IIIA (XTFIIIA): after a decade of research

Transcription factor IIIA (TFIIIA) in the second decade

Transcription factor IIIA is a very extensively studied eukaryotic gene specific factor. It is a special member of the zinc finger family of nucleic acid binding proteins with multiple functions. Its N-terminal polypeptide (280 amino acid residue containing peptide; finger containing region) carries out sequence specific DNA and RNA binding and the C-terminal peptide (65 amino acid residue containing peptide; non-finger region) is involved in the transactivation process possibly by interacting with other general factors. It is a unique factor in the sense that it binds to two structurally different nucleic acids, DNA and RNA. It accomplishes this function through its zinc fingers, which are arranged into a cluster of nine motifs. Over the past three years there has been considerable interest in determining the structural features of zinc fingers, identifying the fingers that preferentially recognize DNA and RNA, defining the role of metal binding ligands and the linker region in promotor recognition and the role of C-terminal amino acid sequence in the gene activation. This article briefly reviews our current knowledge on this special protein in these areas.

Cloning and expression analysis of a human cDNA homologous to Xenopus TFIIIA

We report here the nucleotide sequence of a clone, C2H2-34.10, isolated from a human brain cDNA library using degenerate oligodeoxyribonucleotide hybridization. C2H2-34.10 has extensive homology to the Xenopus laevis 5S DNA/RNA-binding protein, TFIIIA. The deduced amino acid (aa) sequence of the human clone gives a protein of 363 aa with identity to TFIIIA from both X. laevis (57%) and Rana pipiens (59%). This human clone contains nine C2H2-type zinc fingers like frog TFIIIA. Northern blot analysis indicates that the C2H2-34.10 RNA is expressed in human ovary, as well as human neuronal cell lines.

Transcription factor IIIA (TFIIIA): an update

Xenopus transcription factor, termed TFIIIA, is the first eukaryotic transcription factor purified to homogeneity and one of the most extensively characterized polymerase III gene factors at the levels both of the protein and its gene. It is an abundant protein in oocytes and is specifically required for the 5S RNA gene transcription. It promotes the formation of a stable transcription complex by first binding to the internal control region of the 5S RNA gene through its zinc finger motifs. It contains two structural domains and associates with 5S RNA to form 7S ribonucleoprotein particles in oocytes. Its expression is developmentally controlled at the level of transcription and translation. It participates in the assembly of active chromatin templates and, at least in part, is responsible for the differential expression of two kinds of 5S RNA genes in Xenopus.

A novel repression module, an extensive activation domain, and a bipartite nuclear localization signal defined in the immediate-early transcription factor Egr-1

Egr-1 is an immediate-early response gene induced transiently and ubiquitously by mitogenic stimuli and also regulated in response to signals that initiate differentiation. The Egr-1 gene product, a nuclear phosphoprotein with three zinc fingers of the Cys2His2 class, binds to the sequence CGCCCCCGC and transactivates a synthetic promoter construct 10-fold in transient-transfection assays. We have analyzed the structure and function of the Egr-1 protein in detail, delineating independent and modular activation, repression, DNA-binding, and nuclear localization activities. Deletion analysis, as well as fusions to the DNA-binding domain of GAL4, indicated that the activation potential of Egr-1 is distributed over an extensive serine/threonine-rich N-terminal domain. In addition, a novel negative regulatory function has been precisely mapped 5' of the zinc fingers: amino acids 281 to 314 are sufficient to confer the ability to repress transcription on a heterologous DNA-binding domain. Specific DNA-binding activity was shown to reside in the three zinc fingers of Egr-1, as predicted by homology to other known DNA-binding proteins. Finally, nuclear localization of Egr-1 is specified by signals in the DNA-binding domain and basic flanking sequences, as determined by subcellular fractionation and indirect immunofluorescence. Basic residues 315 to 330 confer partial nuclear localization on the bacterial protein beta-galactosidase. A bipartite signal consisting of this basic region in conjunction with either the second or third zinc finger, but not the first, suffices to target beta-galactosidase exclusively to the nucleus. Our work shows that Egr-1 is a functionally complex protein and suggests that it may play different roles in the diverse settings in which it is induced.

A novel repression module, an extensive activation domain, and a bipartite nuclear localization signal defined in the immediate-early transcription factor Egr-1

Egr-1 is an immediate-early response gene induced transiently and ubiquitously by mitogenic stimuli and also regulated in response to signals that initiate differentiation. The Egr-1 gene product, a nuclear phosphoprotein with three zinc fingers of the Cys2His2 class, binds to the sequence CGCCCCCGC and transactivates a synthetic promoter construct 10-fold in transient-transfection assays. We have analyzed the structure and function of the Egr-1 protein in detail, delineating independent and modular activation, repression, DNA-binding, and nuclear localization activities. Deletion analysis, as well as fusions to the DNA-binding domain of GAL4, indicated that the activation potential of Egr-1 is distributed over an extensive serine/threonine-rich N-terminal domain. In addition, a novel negative regulatory function has been precisely mapped 5' of the zinc fingers: amino acids 281 to 314 are sufficient to confer the ability to repress transcription on a heterologous DNA-binding domain. Specific DNA-binding activity was shown to reside in the three zinc fingers of Egr-1, as predicted by homology to other known DNA-binding proteins. Finally, nuclear localization of Egr-1 is specified by signals in the DNA-binding domain and basic flanking sequences, as determined by subcellular fractionation and indirect immunofluorescence. Basic residues 315 to 330 confer partial nuclear localization on the bacterial protein beta-galactosidase. A bipartite signal consisting of this basic region in conjunction with either the second or third zinc finger, but not the first, suffices to target beta-galactosidase exclusively to the nucleus. Our work shows that Egr-1 is a functionally complex protein and suggests that it may play different roles in the diverse settings in which it is induced.

Studies on class III gene transcription

In an attempt to understand the functional role of individual transcription factors, we have carried out S1 nuclease analyses of in vitro synthesized RNA using partially purified HeLa cell factors and truncated 5S and tDNA. The results indicate that faithful initiation of 5S gene transcription depends on the presence of IIIA, IIIB and IIIC in addition to RNA polymerase III. On the other hand, tDNA transcription initiation requires IIIB and IIIC and RNA polymerase III. In addition, we found that either 'A' block promoter or 'B' block alone can not serve as a template for the initiation of tDNA transcription. When an ultraviolet irradiated 5S DNA was used as a template, transcription is minimally affected at a lower dose suggesting that small differences in the helical structure may not have much effect on the transcription machinery. These results along with an in vitro attempt to reproduce Xenopus 5S RNA gene regulation are presented.

The interactions of zinc, nickel, and cadmium with Xenopus transcription factor IIIA, assessed by equilibrium dialysis

Transcription factor IIIA (TFIIIA) was isolated from Xenopus ovary and treated with 1,10-phenanthroline to remove zinc. The interactions of apoTFIIIA with Zn2+, Ni2+, and Cd2+ were studied by equilibrium dialysis under anaerobic conditions (pH 7.0, 25 degrees C), using 65ZnCl2, 63NiCl2, and 109CdCl2 as the radioligands. The data for binding of Zn2+, Ni2+, and Cd2+ to apoTFIIIA were best-fitted by a model with two classes of binding sites. For Zn2+, the apparent dissociation constants (KdlZn and Kd2Zn) for the high- and low-affinity sites were 1.0 x 10(-8) and 2.6 x 10(-5) M; the apparent binding capacities of the two classes were 0.8 +/- 0.5 and 9.6 +/- 0.3 g-atoms of Zn/mol; the Hill coefficient was 1.18, consistent with positive cooperativity of Zn-binding sites. For Ni2+, the apparent KdlNi and Kd2Ni values were 2.3 x 10(-5) and 5.2 x 10(-4) M; the apparent binding capacities were 2.3 +/- 0.6 and 8.6 +/- 0.6 g-atoms of Ni/mol; the Hill coefficient was 1.20, consistent with positive cooperativity of Ni-binding sites. For Cd2+, the apparent KdlCd and Kd2Cd values were 2.8 x 10(-6) and 1.6 x 10(-4) M; the apparent binding capacities were 0.9 +/- 0.3 and 2.4 +/- 0.5 g-atoms of Cd/mol; the Hill coefficient was 0.53, consistent with negative cooperativity or heterogeneity of Cd-binding sites. This study has the following significance: First, it helps to resolve a controversy about the zinc content of purified TFIIIA. Second, it shows that the KdlZn of apoTFIIIA is less than the reported KdZn of thionein, consistent with the hypothesis that thionein modulates gene expression by competing with TFIIIA and other Zn-finger proteins for intracellular Zn2+ stores. Third, it confirms previous indirect evidence that the affinity of apoTFIIIA for Zn2+ is much greater than for Cd2+, and that the affinity for Cd2+ is greater than for Ni2+.