Energetically unfavorable interactions among the zinc fingers of transcription factor IIIA when bound to the 5 S rRNA gene

Contribution of individual amino acids to the RNA binding activity of the Wilms' tumor suppressor protein WT1

In addition to binding to DNA, the zinc finger protein WT1 can also bind specifically to RNA. To determine the role of individual zinc fingers of the protein in this RNA binding activity, deletion and substitution mutants of the WT1 zinc finger domain were constructed. The effects of the various mutations on the binding of WT1 to the RNA aptamers RNA22 and RNA38 were determined using a quantitative equilibrium binding assay. The results indicate that zinc fingers 2 and 3 of WT1 are essential for the binding of the protein to the RNA aptamers. For both of these fingers, the arginine residue immediately preceding the alpha-helix makes a significant contribution to RNA binding. For zinc finger 2, a second arginine residue within the alpha-helix is also critical for RNA binding, while several alpha-helical residues in zinc finger 3 contribute to the overall affinity of WT1 for RNA. Investigating the effects of the same point mutations on DNA binding indicates that there are similarities and differences in the contributions of zinc fingers 2 and 3 to the DNA and RNA binding activities of WT1.

Zinc fingers 1 and 7 of yeast TFIIIA are essential for assembly of a functional transcription complex on the 5 S RNA gene

The binding of transcription factor (TF) IIIA to the internal control region of the 5 S RNA gene is the first step in the assembly of a DNA–TFIIIA–TFIIIC– TFIIIB transcription complex, which promotes accurate transcription by RNA polymerase III. With the use of mutations that are predicted to disrupt the folding of a zinc finger, we have examined the roles of zinc fingers 1 through 7 of yeast TFIIIA in the establishment of a functional transcription complex both in vitro and in vivo. Our data indicate that, in addition to their role in DNA binding, the first and seventh zinc fingers contribute other essential roles in the assembly of an active transcription complex. Alanine-scanning mutagenesis identified residues within zinc finger 1 that are not required for DNA binding but are required for incorporation of TFIIIC into the TFIIIA–DNA complex. Although disruption of zinc finger 2 or 3 had a deleterious effect on the activity of TFIIIA both in vitro and in vivo, we found that increasing the level of their in vivo expression allowed these mutant proteins to support cell viability. Disruption of zinc fingers 4, 5 or 6 had minimal effect on the DNA binding and TF activities of TFIIIA.

Mutations in TFIIIA that increase stability of the TFIIIA-5 S rRNA gene complex: unusual effects on the kinetics of complex assembly and dissociation

We have identified four mutations in Xenopus TFIIIA that increase the stability of TFIIIA-5 S rRNA gene complexes. In each case, the mutation has a relatively modest effect on equilibrium binding affinity. In three cases, these equilibrium binding effects can be ascribed primarily to decreases in the rate constant for protein-DNA complex dissociation. In the fourth case, however, a substitution of phenylalanine for the wild-type leucine at position 148 in TFIIIA results in much larger compensating changes in the kinetics of complex assembly and dissociation. The data support a model in which a relatively unstable population of complexes with multi-component dissociation kinetics forms rapidly; complexes then undergo a slow conformational change that results in very stable, kinetically homogeneous TFIIIA-DNA complexes. The L148F mutant protein acts as a particularly potent transcriptional activator when it is fused to the VP16 activation domain and expressed in yeast cells. Substitution of L148 to tyrosine or tryptophan produces an equally strong transcriptional activator. Substitution to histidine results in genetic and biochemical effects that are more modest than, but similar to, those observed with the L148F mutation. We propose that an amino acid with a planar side chain at position 148 can intercalate between adjacent base pairs in the intermediate element of the 5 S rRNA gene. Intercalation occurs slowly but results in a very stable DNA-protein complex. These results suggest that transcriptional activation by a cis-acting sequence element is largely dependent on the kinetic, rather than the thermodynamic, stability of the complex formed with an activator protein. Thus, transcriptional activation is dependent in large part on the lifetime of the activator-DNA complex rather than on binding site occupancy at steady state. Introduction of intercalating amino acids into zinc finger proteins may be a useful tool for producing artificial transcription factors with particularly high in vivo activity.

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.

Solvation and the hidden thermodynamics of a zinc finger probed by nonstandard repair of a protein crevice

The classical Zn finger contains a phenylalanine at the crux of its three architectural elements: a beta-hairpin, an alpha-helix, and a Zn(2+)-binding site. Surprisingly, phenylalanine is not required for high-affinity Zn2+ binding, but instead contributes to the specification of a precise DNA-binding surface. Substitution of phenylalanine by leucine leads to a floppy but native-like structure whose Zn affinity is maintained by marked entropy-enthalpy compensation (DeltaDeltaH -8.3 kcal/mol and -TDeltaDeltaS 7.7 kcal/mol). Phenylalanine and leucine differ in shape, size, and aromaticity. To distinguish which features correlate with dynamic stability, we have investigated a nonstandard finger containing cyclohexanylalanine at this site. The structure of the nonstandard finger is similar to that of the native domain. The cyclohexanyl ring assumes a chair conformation, and conformational fluctuations characteristic of the leucine variant are damped. Although the nonstandard finger exhibits a lower affinity for Zn2+ than does the native domain (DeltaDeltaG -1.2 kcal/mol), leucine-associated perturbations in enthalpy and entropy are almost completely attenuated (DeltaDeltaH -0.7 kcal/mol and -TDeltaDeltaS -0.5 kcal/mol). Strikingly, global changes in entropy (as inferred from calorimetry) are in each case opposite in sign from changes in configurational entropy (as inferred from NMR). This seeming paradox suggests that enthalpy-entropy compensation is dominated by solvent reorganization rather than nominal molecular properties. Together, these results demonstrate that dynamic and thermodynamic perturbations correlate with formation or repair of a solvated packing defect rather than type of physical interaction (aromatic or aliphatic) within the core.

Restricted specificity of Xenopus TFIIIA for transcription of somatic 5S rRNA genes

Xenopus transcription factor IIIA (TFIIIA) is phosphorylated on serine-16 by CK2. Replacements with alanine or glutamic acid were made at this position in order to address the question of whether phosphorylation possibly influences the function of this factor. Neither substitution has an effect on the DNA or RNA binding activity of TFIIIA. The wild-type factor and the alanine variant activate transcription of somatic- and oocyte-type 5S rRNA genes in nuclear extract immunodepleted of endogenous TFIIIA. The glutamic acid variant (S16E) supports the transcription of somatic-type genes at levels comparable to those of wild-type TFIIIA; however, there is no transcription of the oocyte-type genes. This differential behavior of the phosphomimetic mutant protein is also observed in vivo when using early-stage embryos, where this mutant failed to activate transcription of the endogenous oocyte-type genes. Template exclusion assays establish that the S16E mutant binds to the oocyte-type 5S rRNA genes and recruits at least one other polymerase III transcription factor into an inactive complex. Phosphorylation of TFIIIA by CK2 may allow the factor to continue to act as a positive activator of the somatic-type genes and simultaneously as a repressor of the oocyte-type 5S rRNA genes, indicating that there is a mechanism that actively promotes repression of the oocyte-type genes at the end of oogenesis.

Tropomodulin contains two actin filament pointed end-capping domains

Tropomodulin 1 (Tmod1) is a approximately 40-kDa tropomyosin binding and actin filament pointed end-capping protein that regulates pointed end dynamics and controls thin filament length in striated muscle. In vitro, the capping affinity of Tmod1 for tropomyosin-actin filaments (Kd approximately 50 pm) is several thousand-fold greater than for capping of pure actin filaments (Kd approximately 0.1 microM). The tropomyosin-binding region of Tmod1 has been localized to the amino-terminal portion between residues 1 and 130, but the location of the actin-capping domain is not known. We have now identified two distinct actin-capping regions on Tmod1 by testing a series of recombinant Tmod1 fragments for their ability to inhibit actin elongation from gelsolin-actin seeds using pyrene-actin polymerization assays. The carboxyl-terminal portion of Tmod1 (residues 160-359) contains the principal actin-capping activity (Kd approximately 0.4 microM), requiring residues between 323 and 359 for full activity, whereas the amino-terminal portion of Tmod1 (residues 1-130) contains a second, weaker actin-capping activity (Kd approximately 1.8 microM). Interestingly, 160-359 but not 1-130 enhances spontaneous actin nucleation, suggesting that the carboxyl-terminal domain may bind to two actin subunits across the actin helix at the pointed end, whereas the amino-terminal domain may bind to only one actin subunit. On the other hand, the actin-capping activity of the amino-terminal but not the carboxyl-terminal portion of Tmod1 is enhanced several thousand-fold in the presence of skeletal muscle tropomyosin. We conclude that the carboxyl-terminal capping domain of Tmod1 contains a TM-independent actin pointed end-capping activity, whereas the amino-terminal domain contains a TM-regulated pointed end actin-capping activity.

The hidden thermodynamics of a zinc finger

The Zn finger provides a model for studies of protein structure and stability. Its core contains a conserved phenylalanine residue adjoining three architectural elements: a beta-hairpin, an alpha-helix and a tetrahedral Zn(2+)-binding site. Here, we demonstrate that the consensus Phe is not required for high-affinity Zn(2+) binding but contributes to the specification of a precise DNA-binding surface. Substitution of Phe by leucine in a ZFY peptide permits Zn(2+)-dependent folding. Although a native-like structure is retained, structural fluctuations lead to attenuation of selected nuclear Overhauser enhancements and accelerated amide proton exchange. Surprisingly, wild-type Zn affinity is maintained by entropy-enthalpy compensation (EEC): a hidden entropy penalty (TDeltaDeltaS 7kcal/mol) is balanced by enhanced enthalpy of association (DeltaDeltaH -7kcal/mol) at 25 degrees C. Because the variant is less well ordered than the Phe-anchored domain, the net change in entropy is opposite to the apparent change in configurational entropy. By analogy to the thermodynamics of organometallic complexation, we propose that EEC arises from differences in solvent reorganization. Exclusion of Leu among biological sequences suggests an evolutionary constraint on the dynamics of a Zn finger.

Contribution of Individual Amino Acids to the Nucleic Acid Binding Activities of the Xenopus Zinc Finger Proteins TFIIIIA and p43

Xenopus transcription factor IIIIA (TFIIIA) binds to both 5S RNA and the 5S RNA gene in immature oocytes, an interaction mediated by nine zinc fingers. To determine the role of the central zinc fingers of the protein in these nucleic acid binding activities, a series of substitution mutants of TFIIIA were constructed and expressed as recombinant proteins in Escherichia coli. The mutant proteins were purified to homogeneity and analyzed for DNA and RNA binding activities using a nitrocellulose filter binding assay. Finger 5, but not finger 4, 6, or 7, is involved in sequence-specific binding to the 5S RNA gene. A TWT amino acid motif in finger 6 makes a significant contribution to the binding of TFIIIA to 5S RNA, while mutations in fingers 4, 5, and 7 have little or no effect on RNA binding by TFIIIA. In striking contrast, a TWT motif in finger 6 of p43, another Xenopus zinc finger protein that binds to 5S RNA, is not necessary for 5S RNA binding by this protein. Evidence for the presence of inhibitory finger-finger interactions that limit the nucleic acid binding properties of individual zinc fingers within TFIIIA and p43 is discussed.

The role of the central zinc fingers of transcription factor IIIA in binding to 5 S RNA

In the nine-zinc finger Xenopus transcription factor TFIIIA the central group of fingers, fingers 4 to 7, have been shown to bind to 5 S RNA. In this study, we have attempted to assess the role of this region of the TFIIIA molecule in more detail than hitherto. High-resolution footprinting by RNases A and CV1 has been used to probe the binding to 5 S RNA of three TFIIIA peptides Tf(1-6), Tf(4-6) and Tf(4-7), consisting of fingers 1 to 6, 4 to 6, and 4 to 7, respectively, and of full-length TFIIIA. The results pinpoint the outer margins of binding of the central fingers within helices IV and II of TFIIIA. A comparison of the footprints reveals that the presence of finger 7 affords protection at positions C19 and U55, the distal portion of helix II and the proximal portion of loop B. In addition, our footprints suggest that the central fingers bind in the same manner, whether in an isolated group or in the intact TFIIIA molecule. In a companion study, we have determined the binding affinities of Tf(4-6) and Tf(4-7) for full-length and three truncated 5 S RNA molecules, the latter selected on the basis of the regions of the 5 S RNA molecule known to be important in the binding of TFIIIA. The analysis uses only fully active protein involved in the binding and the results are consistent with the corresponding footprinting results. This is the first time that a detailed study of the binding site of one particular zinc finger to RNA has been reported; the results should be of use in the design of 5 S RNA molecules and TFIIIA peptides for structural studies of the interaction between zinc fingers and RNA.

Identification of a minimal domain of 5 S ribosomal RNA sufficient for high affinity interactions with the RNA-specific zinc fingers of transcription factor IIIA

Transcription factor IIIA of Xenopuslaevis serves a dual function during oogenesis and early development: this zinc finger protein binds to the internal promoter element of the 5 S ribosomal RNA genes and acts as a positive transcription factor; additionally, the protein functions in 5 S RNA storage. The central four zinc fingers (zf4-7) of the nine-finger protein have been shown to bind 5 S rRNA with comparable or higher affinity than the full-length protein. The role of finger seven in binding affinity has been examined by deletion analysis. A zf4-6 protein binds 5 S RNA with about a sevenfold reduction in binding affinity, compared to zf4-7. The effect of non-specific competitor DNA on binding affinities of the zinc finger peptides was examined and found to have a significant effect on the measured affinities of these peptides for full-length and truncated versions of 5 S RNA. The interaction of zf4-6 with full-length 5 S RNA was far more sensitive to non-specific competitor concentration than was the zf4-7:5 S RNA interaction, suggesting that finger seven contributes to both affinity and specificity in this protein:RNA interaction. In order to map zinc finger binding sites on the 5 S RNA molecule, we generated truncated versions of the RNA and tested these molecules for their binding affinities with zf4-7 and zf4-6. Previous studies showed that a 75 nucleotide long RNA, comprising loop A, helix II, helix V, region E and helix IV, bound zf4-7 with high affinity. Selection and amplification binding assays (selex) have now been used to generate smaller high-affinity binding RNAs. We find that a 55 nucleotide long RNA, comprising loop A, helix V, region E and helix IV, but lacking helix II, retains high affinity for zf4-6. These data are consistent with the proposal that fingers 4-6 bind this central core of 5 S RNA and that finger seven binds the helix II region.

Subsets of the zinc finger motifs in dsRBP-ZFa can bind double-stranded RNA

dsRBP-ZFa is a Xenopus zinc finger protein that binds dsRNA and RNA-DNA hybrids with high affinity and in a sequence-independent manner. The protein consists of a basic N-terminal region with seven C2H2 zinc finger motifs and an acidic C-terminal region that is not required for binding. The last four zinc finger motifs, and the linkers that join them, are nearly identical repeats, while the first three motifs and their linkers are each unique. To identify which regions of the protein are involved in nucleic acid binding, we examined the ability of five protein fragments to bind dsRNA and RNA-DNA hybrids. Our studies reveal that a fragment encompassing the three N-terminal, unique zinc finger motifs and another encompassing the last three of the nearly identical motifs have binding properties similar to the full-length protein. Since these two fragments do not share zinc finger motifs of the same sequence, dsRBP-ZFa must contain more than one type of zinc finger motif capable of binding dsRNA. As with the full-length protein, ssRNA and DNA do not significantly compete for dsRNA binding by the fragments.

Domain organization and functional properties of yeast transcription factor IIIA species with different zinc stoichiometries

Transcription factor IIIA (TFIIIA) binds to the 5 S rRNA gene through its zinc finger domain and directs the assembly of a multiprotein complex that promotes transcription initiation by RNA polymerase III. Limited proteolysis of TFIIIA forms with different zinc stoichiometries, in combination with DNA binding and in vitro transcription analyses, have been used herein to investigate the domain organization and zinc requirements of Saccharomyces cerevisiae TFIIIA. Species containing either nine, six, or three zinc equivalents were produced by reductive resaturation and controlled metal depletion of recombinant TFIIIA. Partial digestion of the metal-saturated, 9 Zn2+-liganded factor yields a stable intermediate comprising the eight N-terminal zinc fingers, and a less stable fragment corresponding to a C-terminal portion including the ninth finger. Proteolyzed TFIIIA has the same 5 S DNA binding ability of the intact protein yet no longer supports in vitro 5 S rRNA synthesis. Both the structural compactness and the 5 S DNA binding ability of the TFIIIA form only containing 3 zinc ions are severely compromised. In contrast, the 6 Zn2+-liganded species was found to be indistinguishable from metal-saturated TFIIIA. By demonstrating the existence of three classes of zinc-binding sites contributing differently to yeast TFIIIA structure and function, the present study provides new evidence for the remarkable flexibility built into this complex transcription factor.

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.

Sequence specificity of a group II intron ribozyme: multiple mechanisms for promoting unusually high discrimination against mismatched targets

Group II intron ai5 gamma was reconstructed into a multiple-turnover ribozyme that efficiently cleaves small oligonucleotide substrates in-trans. This construct makes it possible to investigate sequence specificity, since second-order rate constants (kcat/K(m), or the specificity constant) can be obtained and compared with values for mutant substrates and with other ribozymes. The ribozyme used in this study consists of intron domains 1 and 3 connected in-cis, together with domain 5 as a separate catalytic cofactor. This ribozyme has mechanistic features similar to the first step of reverse-splicing, in which a lariat intron attacks exogenous RNA and DNA substrates, and it therefore serves as a model for the sequence specificity of group II intron mobility. To quantitatively evaluate the sequence specificity of this ribozyme, the WT kcat/Km value was compared to individual kcat/Km values for a series of mutant substrates and ribozymes containing single base changes, which were designed to create mismatches at varying positions along the two ribozyme-substrate recognition helices. These mismatches had remarkably large effects on the discrimination index (1/relative kcat/K(m)), resulting in values > 10,000 in several cases. The delta delta G++ for mismatches ranged from 2 to 6 kcal/mol depending on the mismatch and its position. The high specificity of the ribozyme is attributable to effects on duplex stabilization (1-3 kcal/mol) and unexpectedly large effects on the chemical step of reaction (0.5-2.5 kcal/mol). In addition, substrate association is accompanied by an energetic penalty that lowers the overall binding energy between ribozyme and substrate, thereby causing the off-rate to be faster than the rate of catalysis and resulting in high specificity for the cleavage of long target sequences (> or = 13 nucleotides).