Contribution of individual base pairs to the interaction of TFIIIA with the Xenopus 5S RNA gene

Variation in the Karyotype, Cytochrome b Gene, and 5S rDNA of Four Thunnus (Perciformes, Scombridae) Tunas

Yan-Horn Lee, Tsair-Bor Yen, Chiu-Fen Chen, and Mei-Chen Tseng (2018) Thunnus tunas in Scombridae are divided into the temperate subgenus Thunnus (bluefin group) and tropical subgenus Neothunnus (yellowfin group) species based on anatomic traits and distributions. The main purpose of this study was to examine the systematic status of T. obesus based on karyotype, cytochrome (Cyt) b gene, and 5S ribosomal DNA sequences. All T. obesus, T. albacares, T. alalunga, and T. orientalis specimens were caught in southeastern coastal waters off the main island of Taiwan. The karyotypical formula of T. obesus was 2 m + 2 st + 44 t, that of T. albacares was 2 m + 2 sm + 2 st + 42 t, that of T. alalunga was 2 m + 2 sm + 2 st + 42 t, and that of T. orientalis was 2 m + 2 sm + 44 t (m: metacentric; sm: submetacentric; st: subtelocentric; t: telocentric chromosome). According to a molecular genetics analysis for these species using Cyt b gene sequences (1141 bp), interspecific genetic distances ranged from 0.004 (T. orientalis vs. T. alalunga) to 0.038 (T. alalunga vs. T. obesus). The genealogy tree exhibited these 4 species as being categorized into 4 monophyletic groups with high bootstrapping values; T. alalunga and T. orientalis are sister species. This result suggests that the species currently allocated in Thunnus and Neothunnus might need new taxonomic characters to redefine the monophyly of the two subgenera. The sequence lengths of all cloned 5S genes from the 4 species ranged from 327-342 bp. Interspecific genetic distances ranged from 0.016 (T. orientalis vs. T. alalunga) to 0.111 (T. orientalis vs. T. albacares). The phylogenetic tree based on 5S rDNA shows T. obesus divided into 2 groups: one similar to T. albacares and the other close to T. orientalis. These results imply that Thunnus tunas have a common synapomorphic character with Scombridae fish (2n = 48) and high numbers of telocentric chromosomes (42-44). Thunnus orientalis and T. alalunga are sister based on molecular data. Thunnus obesus may have been derived from a more-complicated speciation processes.

Evidence of birth-and-death evolution of 5S rRNA gene in Channa species (Teleostei, Perciformes)

In higher eukaryotes, minor rDNA family codes for 5S rRNA that is arranged in tandem arrays and comprises of a highly conserved 120 bp long coding sequence with a variable non-transcribed spacer (NTS). Initially the 5S rDNA repeats are considered to be evolved by the process of concerted evolution. But some recent reports, including teleost fishes suggested that evolution of 5S rDNA repeat does not fit into the concerted evolution model and evolution of 5S rDNA family may be explained by a birth-and-death evolution model. In order to study the mode of evolution of 5S rDNA repeats in Perciformes fish species, nucleotide sequence and molecular organization of five species of genus Channa were analyzed in the present study. Molecular analyses revealed several variants of 5S rDNA repeats (four types of NTS) and networks created by a neighbor net algorithm for each type of sequences (I, II, III and IV) did not show a clear clustering in species specific manner. The stable secondary structure is predicted and upstream and downstream conserved regulatory elements were characterized. Sequence analyses also shown the presence of two putative pseudogenes in Channa marulius. Present study supported that 5S rDNA repeats in genus Channa were evolved under the process of birth-and-death.

Contributions of in vitro transcription to the understanding of human RNA polymerase III transcription

Human RNA polymerase III transcribes small untranslated RNAs that contribute to the regulation of essential cellular processes, including transcription, RNA processing and translation. Analysis of this transcription system by in vitro transcription techniques has largely contributed to the discovery of its transcription factors and to the understanding of the regulation of human RNA polymerase III transcription. Here we review some of the key steps that led to the identification of transcription factors and to the definition of minimal promoter sequences for human RNA polymerase III transcription.

Classification of a frameshift/extended and a stop mutation in WT1 as gain-of-function mutations that activate cell cycle genes and promote Wilms tumour cell proliferation

The WT1 gene encodes a zinc finger transcription factor important for normal kidney development. WT1 is a suppressor for Wilms tumour development and an oncogene for diverse malignant tumours. We recently established cell lines from primary Wilms tumours with different WT1 mutations. To investigate the function of mutant WT1 proteins, we performed WT1 knockdown experiments in cell lines with a frameshift/extension (p.V432fsX87 = Wilms3) and a stop mutation (p.P362X = Wilms2) of WT1, followed by genome-wide gene expression analysis. We also expressed wild-type and mutant WT1 proteins in human mesenchymal stem cells and established gene expression profiles. A detailed analysis of gene expression data enabled us to classify the WT1 mutations as gain-of-function mutations. The mutant WT1^Wilms2 and WT1^Wilms3 proteins acquired an ability to modulate the expression of a highly significant number of genes from the G2/M phase of the cell cycle, and WT1 knockdown experiments showed that they are required for Wilms tumour cell proliferation. p53 negatively regulates the activity of a large number of these genes that are also part of a core proliferation cluster in diverse human cancers. Our data strongly suggest that mutant WT1 proteins facilitate expression of these cell cycle genes by antagonizing transcriptional repression mediated by p53. We show that mutant WT1 can physically interact with p53. Together the findings show for the first time that mutant WT1 proteins have a gain-of-function and act as oncogenes for Wilms tumour development by regulating Wilms tumour cell proliferation.

An RNA aptamer with high affinity and broad specificity for zinc finger proteins

A class of RNA aptamers that demonstrates a high affinity for a large variety of C(2)H(2) zinc finger proteins was isolated from a library of random RNA sequences by the zinc finger protein TFIIIA. These aptamers have one or more copies of the consensus sequence GGGUGGG, which is part of a putative hairpin loop in the proposed structure of the most abundant aptamer, RNA1. Binding of zinc finger proteins to RNA1 relies upon zinc-dependent folding of the protein, the affinity of an individual protein for RNA1 being determined by the number of tandem zinc finger motifs. The properties of RNA1 were compared to the properties of two other aptamers from the same selection experiment: RNA21, which binds to some but not all zinc finger proteins tested, and RNA22, which binds only to the 5S rRNA binding zinc finger proteins TFIIIA and p43. The binding of three different zinc finger proteins to RNA1 was compared, and the results indicate that the RNA1-protein interaction occurs by several distinct mechanisms. Mutagenesis of RNA1 confirmed that the GGGUGGG consensus sequence presented in a hairpin conformation is required for high-affinity binding of zinc finger proteins.

An RNA aptamer with high affinity and high specificity for the 5S RNA binding zinc finger proteins TFIIIA and p43

The Xenopus zinc finger proteins TFIIIA and p43 bind to 5S RNA in immature oocytes to form 7S and 42S ribonucleoprotein storage particles. To probe the similarities and differences in the RNA binding domains of these two proteins, a library of random RNA molecules was enriched using TFIIIA as the bait protein. One of the abundant aptamers isolated, RNA22, bound to both TFIIIA and p43 derived zinc finger peptides with high affinity and specificity even though the predicted secondary structure of the RNA was unrelated to that of 5S RNA. The interactions of TFIIIA and p43 peptides with RNA22 were compared to their interactions with 5S RNA by characterizing the effects of assay conditions, mutations in RNA22, and mutations in the zinc finger proteins. The similarities and differences in the mechanisms by which these two zinc finger proteins interact with 5S RNA compared to RNA22 suggest they share a common platform for RNA binding with enough flexibility to form specific interactions with both RNAs.

Is There a Dynamic DNA-Protein Interface in the Transcription Factor IIIA-5 S rRNA Gene Complex?

Others have proposed that several amino acid side chains exhibit considerable conformational mobility at the DNA-protein interface in the transcription factor IIIA.5 S rRNA gene complex and that the rapid movements of these side chains permit them to make fluctuating contacts with adjacent bp in the DNA target site. This "dynamic interface" model makes biochemical predictions concerning the consequences of truncating specific amino acid side chains and the effects of these truncations on sequence selectivity in DNA binding. The model also makes predictions concerning the effects of DNA sequence context on the apparent energetic contributions to binding made by individual bp. We have tested these predictions, and our results are inconsistent with any significant energetic role being played by the contact of multiple bp by conformationally mobile amino acid side chains. They do, however, show that some individual amino acids affect the recognition of multiple bp through mechanisms other than direct interaction.

Binding of zinc finger protein transcription factor IIIA to its cognate DNA sequence with single UV photoproducts at specific sites and its effect on DNA repair

The relationship between DNA repair efficiency at specific locations in the binding site of the nine-zinc finger protein transcription factor IIIA (TFIIIA) and binding of its individual zinc fingers was studied. Homogeneously damaged oligonucleotides, which contained a single cis-syn cyclobutane thymine dimer (CTD) at one of six different sites in the internal control region (ICR) of the 5 S rRNA gene to generate a series of damaged DNA substrates, were prepared by chemical synthesis. Binding of TFIIIA to the substrates was assayed by measurement of dissociation constants (Kd), dissociation rates (koff), and protein-DNA contacts. The results indicated that a single CTD in the ICR does not significantly affect the Kd of TFIIIA. In contrast, CTDs at positions +55 and +72 (from the transcription start site) in the ICR markedly enhanced koff of TFIIIA from the complex. In addition, CTDs in these two sites increased methylation of the N7 of guanines (by dimethyl sulfate) in the zinc finger contacts of the ICR-TFIIIA complex. Furthermore CTDs at +55 and +72 were more efficiently removed from the complex than CTDs at other sites in the ICR by Xenopus oocyte nuclear extracts. This suggests that repair of CTDs closely correlates with changes in the binding of individual zinc fingers of the ICR-TFIIIA complex. These results have implications for the mechanism of DNA damage recognition and repair in protein-DNA complexes.

Effect of arginine mutation of alanine-556 on DNA recognition of zinc finger protein Sp1

Human transcription factor Sp1, which contains three Cys(2)His(2)-class zinc finger motives, specifically binds to the so-called GC box DNA. It has been indicated that finger 1 has a unique DNA-binding mode compared with fingers 2 and 3, or the Zif268 model. Therefore, we investigate the role of Ala at position 6 on the recognition helix, which is not responsible for guanine recognition and highly conserved among Sp1 family. Several Ala-556 mutations of Sp1 bind to DNA with different DNA-binding features. In particular, the Ala-->Arg substitution alters the DNA-binding contribution of the three zinc fingers in Sp1. In this case, the DNA-binding specificity of each finger decreases in the order 2>1>3. This result reveals that one amino acid in position 6 plays an important role not only for the selectivity to the putative finger 1 subsite, but also for the binding mode of the three fingers to each finger subsite. Probably, Ala-556 is indispensable to characterize the binding mode of the Sp1 zinc fingers, namely the diverse binding contribution of finger 1 and the rigid binding one of finger 3. In Sp1, the N-terminal finger 1 serves as a 'hinge finger'.

NMR and molecular dynamics studies of the hydration of a zinc finger-DNA complex

The hydration of a high-affinity protein-DNA complex involving the three amino terminal zinc finger domains of transcription factor IIIA (TFIIIA) and a 15-base-pair DNA duplex was investigated by NMR spectroscopy and molecular dynamics (MD) simulations. Intermolecular nuclear Overhauser effects (NOEs) between protein and water provided an experimental basis for identifying potential sites of hydration. These initial assignments were evaluated with the aid of two, 2 ns MD simulations of the protein-DNA complex conducted with the explicit inclusion of water solvent. The two independent simulations produced similar trends in terms of water residence times around the solute, and these results were used to separate protein-water NOEs from alternate exchange-relayed cross peaks. Furthermore, only six of the 170 protons which failed to show intermolecular NOEs to solvent showed nearby long-resident water molecules in the MD simulations, illustrating an impressive level of agreement between theory and experiment. Analyses of the MD trajectories also allowed an examination of the role of water in recognition and binding affinity of the zinc fingers with DNA. The interface is well hydrated, characterized by direct contacts between the protein and DNA, as well as mediating water bridges. Approximately 18 water-mediated hydrogen bonds between the protein and DNA were observed on average. Roughly half of these were water molecules with long residence times that are most likely to be important for binding, since they involve residues which have been shown through biochemical studies to be crucial for protein-DNA binding. This level of atomic detail could not otherwise be established through the existing NMR and crystal structures of the TFIIIA-DNA complex.

Genetic analysis of Xenopus transcription factor IIIA

We describe a method for the genetic analysis of the DNA-binding properties of Xenopus transcription factor IIIA (TFIIIA). In this approach, a transcriptional activator with the DNA-binding specificity of Xenopus TFIIIA is expressed in yeast cells, where it specifically activates expression of a beta-galactosidase reporter gene containing one or more Xenopus 5 S rRNA genes that function as upstream activator sequences. This transcription-promoting activity was used as the basis for a genetic assay of Xenopus TFIIIA's DNA-binding function in yeast, an assay that we show can be calibrated quantitatively to allow the affinity of the Xenopus TFIIIA-5 S rRNA gene interaction to be deduced from measurements of beta-galactosidase activity. We have combined this genetic assay with a simple and efficient method of mutagenesis that makes use of error-prone PCR and homologous recombination to generate and screen large numbers of TFIIIA mutants for those with altered 5 S rRNA gene-binding affinity. Over 30 such mutants have been identified and partially characterized. The mutants we have obtained provide strong support for the application to intact TFIIIA of recent structural models of the N-terminal zinc fingers of the protein bound to fragments of the 5 S rRNA gene. Other mutants permit identification of important residues in more C-terminal zinc fingers of TFIIIA for which high-resolution structural information is not currently available. Finally, our results have interesting implications with respect to the mechanism of activation of transcription by RNA polymerase II in yeast.

Differing roles for zinc fingers in DNA recognition: structure of a six-finger transcription factor IIIA complex

The crystal structure of the six NH2-terminal zinc fingers of Xenopus laevis transcription factor IIIA (TFIIIA) bound with 31 bp of the 5S rRNA gene promoter has been determined at 3.1 A resolution. Individual zinc fingers are positioned differently in the major groove and across the minor groove of DNA to span the entire length of the duplex. These results show how TFIIIA can recognize several separated DNA sequences by using fewer fingers than necessary for continuous winding in the major groove.

Nucleosome translational position, not histone acetylation, determines TFIIIA binding to nucleosomal Xenopus laevis 5S rRNA genes

We sought to study the binding constraints placed on the nine-zinc-finger protein transcription factor IIIA (TFIIIA) by a histone octamer. To this end, five overlapping fragments of the Xenopus laevis oocyte and somatic 5S rRNA genes were reconstituted into nucleosomes, and it was subsequently shown that nucleosome translational positioning is a major determinant of the binding of TFIIIA to the 5S rRNA genes. Furthermore, it was found that histone acetylation cannot override the TFIIIA binding constraints imposed by unfavorable translational positions.

Molecular biology of vertebrate transcription factor IIIA: cloning and characterization of TFIIIA from channel catfish oocytes

TFIIIA regulates 5S rRNA synthesis and is the prototype of the Cys2His2 superfamily of zinc finger proteins. Because the TFIIIA aa sequence is highly diverged, elucidating species variation in this factor will yield insights into how zinc fingers bind DNA and how this protein regulates RNAPIII transcription. This study reports the identification, cloning and functional divergence of oocyte TFIIIA from the channel catfish. Catfish oocyte TFIIIA was identified by its association with 5S rRNA in immature ovarian tissue, its molecular weight, and by peptide sequence similarities with Xenopus TFIIIA. The cDNA for this factor was cloned by degenerate PCR and found to code for nine Cys2His2 zinc fingers and a C-terminal tail; only about 40% aa sequence identity was observed with Xenopus TFIIIA. The N-terminal region of catfish TFIIIA contains the oocyte-specific initiating Met amino acid and accompanying conserved residues found in amphibian TFIIIAs but not found in yeast or human TFIIIAs. Catfish TFIIIA lacks the conserved transcription activation domain in its C-terminal tail found in amphibian and human TFIIIA. Catfish TFIIIA was able to bind the catfish and Xenopus 5S RNA genes but did not efficiently promote 5S gene transcription in a rodent RNAPIII transcription system, as did Xenopus TFIIIA. Amino acid conservation in catfish, amphibian, and human TFIIIA zinc fingers allows deduction of possible finger recognition helix alignments along the conserved 5S gene ICRs. For the three N-terminal fingers, this leads to deduction of a compact polypeptide structure with conserved basic residues contacting conserved G nts in the 5S gene C box.

Solution structure of the first three zinc fingers of TFIIIA bound to the cognate DNA sequence: determinants of affinity and sequence specificity

The high resolution solution structure of a protein containing the three amino-terminal zinc fingers of Xenopus laevis transcription factor IIIA (TFIIIA) bound to its cognate DNA duplex was determined by nuclear magnetic resonance spectroscopy. The protein, which is designated zf1-3, binds with all three fingers in the DNA major groove, with a number of amino acids making base-specific contacts. The DNA structure is close to B-form. Although the mode of interaction of zf1-3 with DNA is similar to that of zif268 and other structurally characterized zinc finger complexes, the TFIIIA complex exhibits several novel features. Each zinc finger contacts four to five base-pairs and the repertoire of known base contact residues is extended to include a tryptophan at position +2 of the helix (finger 1) and arginine at position +10 (finger 3). Sequence-specific base contacts are made over virtually the entire length of the finger 3 helix. Lysine and histidine side-chains involved in base recognition are dynamically disordered in the solution structure; in the case of lysine, in particular, this could significantly decrease the entropic cost of DNA binding. The TGEKP(N) linker sequences, which are highly flexible in the unbound protein, adopt ordered conformations on DNA binding. The linkers appear to play an active structural role in stabilization of the protein-DNA complex. Substantial protein-protein contact surfaces are formed between adjacent fingers. As a consequence of these protein-protein interactions, the orientation of finger 1 in the major groove differs from that of the other fingers. Contributions to high affinity binding by zf1-3 come from both direct protein-DNA contacts and from indirect protein-protein interactions associated with structural organization of the linkers and formation of well-packed interfaces between adjacent zinc fingers in the DNA complex. The structures provide a molecular level explanation for the large body of footprinting and mutagenesis data available for the TFIIIA-DNA complex.

Assessment of major and minor groove DNA interactions by the zinc fingers of Xenopus transcription factor IIIA

Zinc finger proteins of the Cys2His2 class are DNA sequence-specific transcription factors. Previous structural studies of zinc finger protein-DNA complexes have shown that amino acids in the finger tip and alpha-helix regions within individual finger domains make base-specific contacts with the major groove of DNA. The nine finger protein transcription factor IIIA (TFIIIA) from Xenopus oocytes binds a 43 base pair region of the 5S RNA gene through major groove interactions with two sets of three fingers (fingers 1-3 and 7-9) and with finger 5. Previous studies have suggested that zinc fingers 4 and 6 each bind in or across the minor groove to bridge these major groove-binding zinc fingers. Here it is shown that a polypeptide containing zinc fingers 1-5 (zf1-5) binds oligonucleotides with modifications in the major groove of the finger 4 binding site with wild-type affinity. Mutagenesis and binding site selection studies were performed to determine whether high affinity DNA binding by zf1-5 requires a particular sequence in the binding site for finger 4. Several mutations in this region of the 5S gene reduced the DNA-binding affinity of zf1-5; however, selection and amplification binding assays did not recover the wild-type finger 4 binding site sequence from a pool of mixed sequence oligonucleotides. Rather, a purine-rich sequence on the top strand was highly selected within the finger 4 binding site. We suggest that high affinity DNA binding by zinc finger 4 may be dictated by a sequence-specific DNA structure rather than by a unique DNA sequence. Deletion of finger 4 from zf1-5 results in a protein with poor binding affinity, demonstrating the importance of finger 4 in proper alignment of neighboring fingers with the DNA, and/or the importance of correct protein-protein interactions between fingers.

Interaction of wild-type and truncated forms of transcription factor IIIA from Saccharomyces cerevisiae with the 5 S RNA gene

Transcription factor (TF) IIIA, which contains nine zinc finger motifs, binds to the internal control region of the 5S RNA gene as the first step in the assembly of a multifactor complex that promotes accurate initiation of transcription by RNA polymerase III. We have monitored the interaction of wild-type and truncated forms of yeast TFIIIA with the 5 S RNA gene. The DNase I footprints obtained with full-length TFIIIA and a polypeptide containing the amino-terminal five zinc fingers (TF5) were indistinguishable, extending from nucleotides +64 to +99 of the 5 S RNA gene. This suggests that fingers 6 through 9 of yeast TFIIIA are not in tight association with DNA. The DNase I footprint obtained with a polypeptide containing the amino-terminal four zinc fingers (TF4) was 14 base pairs shorter than that of TF5, extending from nucleotides +78 to +99 on the nontranscribed strand and from nucleotides +79 to +98 on the transcribed strand of the 5 S RNA gene. Protection provided by a polypeptide containing the first three zinc fingers (TF3) was similar to that provided by TF4, with the exception that protection on the nontranscribed strand ended at nucleotide +80, rather than nucleotide +78. Methylation protection analysis indicated that finger 5 makes major groove contacts with guanines +73 and +74. The amino-terminal four zinc fingers make contacts that span the internal control region, which extends from nucleotides +81 to +94 of the 5 S RNA gene, with finger 4 appearing to contact guanine +82. Measurements of the apparent Kd values of the TFIIIA.DNA complexes indicated that the amino-terminal three zinc fingers of TFIIIA have a binding energy that is similar to that of the full-length protein.

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.

Analysis of the binding of Xenopus transcription factor IIIA to oocyte 5 S rRNA and to the 5 S rRNA gene

Binding of transcription factor IIIA (TFIIIA) to site-specific mutants of Xenopus oocyte 5 S rRNA has been used to identify important recognition elements in the molecule. The putative base triple G75:U76:A100 appears to determine the conformation of the loop E region whose integrity is especially important for binding of the factor. Proximal substitutions in helices IV and V indicate that the proper folding of loop E is also dependent on these structures. Mutations in helix V affect binding of TFIIIA to 5 S rRNA and to the gene similarly and provide evidence that zinc finger 5 makes sequence-specific contact through the major groove of both nucleic acids. Although fingers 1-3 are positioned along helix IV and loop D, mutations in this region, including those that disrupt the tetraloop or close the opening in the major groove of the helix created by the U80:U96 mismatch, have no impact on binding. Substitutions made at stem-loop junctions in the arm of the RNA comprised of helix II-loop B-helix III display minor decreases in affinity for TFIIIA. Despite the alignment of the factor along nearly the entire length of 5 S rRNA, the essential elements for high affinity binding are limited to the central region of the molecule. Analysis of the corresponding mutations in the gene confirm that box C and the intermediate element provide the high affinity sites for binding of the factor to the DNA. Despite the small thermodynamic contribution made by contacts to box A, mutations made in this element can cause substantial changes in the orientation of the carboxyl-terminal fingers along the 5'-end of the internal control region.

The primary structure of Harpalus rufipes 5S ribosomal RNA: a contribution for understanding insect evolution

The nucleotide sequence of 5S ribosomal RNA from the beetle Harpalus rufipes was determined and compared with primary structures of other insect 5S rRNAs. Sequence differences between two beetle 5S rRNAs may represent phylogenetic markers specific for two groups of Coleoptera - Adephaga and Polyphaga. Analysis of all insect sequences using parsimony allowed us to infer a phylogenetic tree of insects, which is consistent with morphological and paleobiological data.

Zinc(II) ions selectively interact with DNA sequences present at the TFIIIA binding site of the Xenopus 5S-RNA gene

It has been known for some time that zinc, as well as most transition metal ions, is capable of binding to the DNA bases. However, little is known about the presence and distribution of metal binding sites along naturally occurring genomic DNA molecules. In this paper, the interaction of zinc with the Xenopus 5S-RNA gene has been studied and several metal binding sites have been identified on the basis of the changes in chemical reactivity observed in the presence of the metal. The strongest zinc-binding sites of the Xenopus 5S-RNA gene correspond to GGG trinucleotide repeats. Some GG dinucleotides also show a significant affinity for zinc. Interestingly, the binding site for TFIIIA, a zinc-finger transcription factor, contains several sites with strong zinc affinity. In particular, a TGGGA sequence which is essential for the binding of TFIIIA shows the strongest affinity for zinc. The conformational properties of this DNA sequence, together with the high electronegative potential of GGG runs, is likely to determine its strong affinity for zinc. The possible biological relevance of these results is discussed.