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L James Maher

Publications and source records attributed to L James Maher.

17 recordsLinked to original sources
PubMed2006

DNA bending by bHLH charge variants.

We wish to understand the role of electrostatics in DNA stiffness and bending. The DNA charge collapse model suggests that mutual electrostatic repulsions between neighboring phosphates significantly contribute to DNA stiffness. According to this model, placement of fixed charges near the negatively charged DNA surface should induce bending through asymmetric reduction or enhancement of these inter-phosphate repulsive forces. We have reported previously that charged variants of the elongated basic-leucine zipper (bZIP) domain of Gcn4p bend DNA in a manner consistent with this charge collapse model. To extend this result to a more globular protein, we present an investigation of the dimeric basic-helix-loop-helix (bHLH) domain of Pho4p. The 62 amino acid bHLH domain has been modified to position charged amino acid residues near one face of the DNA double helix. As observed for bZIP charge variants, DNA bending toward appended cations (away from the protein:DNA interface) is observed. However, unlike bZIP proteins, DNA is not bent away from bHLH anionic charges. This finding can be explained by the structure of the more globular bHLH domain which, in contrast to bZIP proteins, makes extensive DNA contacts along the binding face.

Basic Helix-Loop-Helix Proteins↗
PubMed2006

Recognition of RNA by the p53 tumor suppressor protein in the yeast three-hybrid system.

The p53 tumor suppressor protein is a homotetrameric transcription factor whose gene is mutated in nearly half of all human cancers. In an unrelated screen of RNA/protein interactions using the yeast three-hybrid system, we inadvertently detected p53 interactions with several different RNAs. A literature review revealed previous reports of both sequence-specific and -non-specific interactions between p53 and RNA. Using yeast three-hybrid selections to identify preferred RNA partners for p53, we failed to identify primary RNA sequences or obvious secondary structures required for p53 binding. The cationic p53 C-terminus was shown to be required for RNA binding in yeast. We show that while p53 strongly discriminates between certain RNAs in the yeast three-hybrid assay, the same RNAs are bound equally by p53 in vitro. We further show that the p53 RNA-binding preferences in yeast are mirrored almost exactly by a recombinant tetrameric form of the HIV-1 nucleocapsid (NC) protein thought to be a sequence-nonspecific RNA-binding protein. However, the possibility of specific RNA binding by p53 could not be ruled out because p53 and HIV-1 NC displayed certain differences in RNA-binding preference. We conclude that (1) p53 binds RNA in vivo, (2) RNA binding by p53 is largely sequence-nonspecific in the yeast nucleus, (3) some structure-specific RNA binding by p53 cannot be ruled out, and (4) caution is required when interpreting results of RNA screens in the yeast three-hybrid system because sequence-dependent differences in RNA folding and display can masquerade as sequence-dependent differences in protein recognition.

Base Sequence↗
PubMed2005

Bacterial repression loops require enhanced DNA flexibility.

The Escherichia coli lac operon provides a classic paradigm for understanding regulation of gene transcription. It is now appreciated that lac promoter repression involves cooperative binding of the bidentate lac repressor tetramer to pairs of lac operators via DNA looping. We have adapted components of this system to create an artificial assay of DNA flexibility in E.coli. This approach allows for systematic study of endogenous and exogenous proteins as architectural factors that enhance apparent DNA flexibility in vivo. We show that inducer binding does not completely remove repression loops but it does alter their geometries. Deletion of the E.coli HU protein drastically destabilizes small repression loops, an effect that can be partially overcome by expression of a heterologous mammalian HMG protein. These results emphasize that the inherent torsional inflexibility of DNA restrains looping and must be modulated in vivo.

Animals↗
PubMed2005

Dual binding modes for an HMG domain from human HMGB2 on DNA.

High mobility group B (HMGB) proteins contain two HMG box domains known to bind without sequence specificity into the DNA minor groove, slightly intercalating between basepairs and producing a strong bend in the DNA backbone. We use optical tweezers to measure the forces required to stretch single DNA molecules. Parameters describing DNA flexibility, including contour length and persistence length, are revealed. In the presence of nanomolar concentrations of isolated HMG box A from HMGB2, DNA shows a decrease in its persistence length, where the protein induces an average DNA bend angle of 114 +/- 21 degrees for 50 mM Na+, and 87 +/- 9 degrees for 100 mM Na+. The DNA contour length increases from 0.341 +/- 0.003 to 0.397 +/- 0.012 nm per basepair, independent of salt concentration. In 50 mM Na+, the protein does not unbind even at high DNA extension, whereas in 100 mM Na+, the protein appears to unbind only below concentrations of 2 nM. These observations support a flexible hinge model for noncooperative HMG binding at low protein concentrations. However, at higher protein concentrations, a cooperative filament mode is observed instead of the hinge binding. This mode may be uniquely characterized by this high-force optical tweezers experiment.

Animals↗
PubMed2005

Indirect detection of DNA damage.

In this issue of Chemistry and Biology, Naegeli and coworkers show that the nucleotide excision repair system of mammalian cells detects bulky DNA adducts, not by recognition of the adduct per se, but by recognition of the undamaged partner strand in bulged form.

DNA Adducts↗
PubMed2003

Solution measurement of DNA curvature in papillomavirus E2 binding sites.

'Indirect readout' refers to the proposal that proteins can recognize the intrinsic three-dimensional shape or flexibility of a DNA binding sequence apart from direct protein contact with DNA base pairs. The differing affinities of human papillomavirus (HPV) E2 proteins for different E2 binding sites have been proposed to reflect indirect readout. DNA bending has been observed in X-ray structures of E2 protein-DNA complexes. X-ray structures of three different E2 DNA binding sites revealed differences in intrinsic curvature. DNA sites with intrinsic curvature in the direction of protein-induced bending were bound more tightly by E2 proteins, supporting the indirect readout model. We now report solution measurements of intrinsic DNA curvature for three E2 binding sites using a sensitive electrophoretic phasing assay. Measured E2 site curvature agrees well the predictions of a dinucleotide model and supports an indirect readout hypothesis for DNA recognition by HPV E2.

Algorithms↗
PubMed2003

Crystal structure of NF-kappaB (p50)2 complexed to a high-affinity RNA aptamer.

We have recently identified an RNA aptamer for the transcription factor NF-kappaB p50 homodimer [(p50)2], which exhibits little sequence resemblance to the consensus DNA target for (p50)2, but binds (p50)2 with an affinity similar to that of the optimal DNA target. We describe here the 2.45-A resolution x-ray crystal structure of the p50 RHR/RNA aptamer complex. The structure reveals that two RNA molecules bind independent of each other to the p50 N-terminal Ig-like domains. The RNA secondary structure is comprised of a stem and a stem-loop separated by an internal loop folded into a kinked helix because of the cross-strand stacking of three internal loop guanines. These guanines, placed at the edge of the 3' helix, together with the major groove of the irregular 3' helix, form the binding surface for p50. Each p50 monomer uses the same surface to recognize the distorted RNA major groove as observed in the kappaB DNA/p50 RHR complex, resulting in strikingly similar interfaces. The structure reveals how the aptamer specifically selects p50 and discriminates against p65. We also discuss the physiological implications of RNA binding by (p50)2.

Amino Acid Sequence↗
PubMed2003

Yeast genetic selections to optimize RNA decoys for transcription factor NF-kappa B.

In vitro-selected RNA aptamers are potential inhibitors of disease-related proteins. Our laboratory previously isolated an RNA aptamer that binds with high affinity to human transcription factor NF-kappaB. This RNA aptamer competitively inhibits DNA binding by NF-kappaB in vitro and is recognized by its target protein in vivo in a yeast three-hybrid system. In the present study, yeast genetic selections were used to optimize the RNA aptamer for binding to NF-kappaB in the eukaryotic nucleus. Selection for improved binding to NF-kappaB from RNA libraries encoding (i) degenerate aptamer variants and (ii) sequences present at round 8 of 14 total rounds of in vitro selection yielded RNA aptamers with dramatically improved in vivo activity. Furthermore, we show that an in vivo-optimized RNA aptamer exhibits specific "decoy" activity, inhibiting transcriptional activation by its NF-kappaB target protein in a yeast one-hybrid assay. This decoy activity is enhanced by the expression of a bivalent aptamer. The combination of in vitro and in vivo genetic selections was crucial for obtaining RNA aptamers with in vivo decoy activity.

Base Sequence↗
PubMed2003

Reflections on apparent DNA bending by charge variants of bZIP proteins.

Basic-leucine zipper (bZIP) proteins have been studied intensely as transcription factors. It has been proposed that the bZIP domain might modulate transcription activation through the induction of conformational changes in the DNA binding site. We have been interested in using bZIP peptides as convenient models with which to study the role of asymmetric phosphate neutralization in DNA bending. DNA bending experiments have yielded discordant results for bZIP peptides studied by electrophoretic- vs solution-based assays. We review the history of DNA bending assays involving bZIP peptides and introduce the reader to examples of discordant results. Our recent published experiments designed to clarify this field of study will then be reviewed. The engineering of protein fusions has established that electrophoretic phasing assays are relatively insensitive to precise protein structure/conformation and instead appear to report DNA bending, as influenced by protein charge. New applications of time-resolved fluorescence resonance energy transfer (FRET) have allowed for the first time corroboration of electrophoretic phasing assays with solution-based FRET measurements. We report that two conventional DNA bending assays that rely on DNA ligation cannot be applied to analysis of the bZIP peptides we studied due to ligation inhibition.

Amino Acid Sequence↗
PubMed2002

In vivo selection of spectinomycin-binding RNAs.

The folding of even short RNA molecules in a random library can produce a huge number of possible macromolecular structures. Using this principle, we have designed selections to seek non-coding RNA transcripts capable of interfering with specific macromolecules such as transcription factors in living bacterial cells. Here we show that such selections can uncover an unexpected class of RNAs. In the present case, we report short RNA transcripts whose expression confers bacterial resistance to the antibiotic spectinomycin. We provide evidence that such RNAs cause drug resistance by direct antibiotic binding, demonstrating a class of spectinomycin-specific functional molecular decoys built from RNA.

Anti-Bacterial Agents↗
PubMed2002

Having it both ways: transcription factors that bind DNA and RNA.

Multifunctional proteins challenge the conventional 'one protein-one function' paradigm. Here we note apparent multifunctional proteins with nucleic acid partners, tabulating eight examples. We then focus on eight additional cases of transcription factors that bind double-stranded DNA with sequence specificity, but that also appear to lead alternative lives as RNA-binding proteins. Exemplified by the prototypic Xenopus TFIIIA protein, and more recently by mammalian p53, this list of transcription factors includes WT-1, TRA-1, bicoid, the bacterial sigma(70) subunit, STAT1 and TLS/FUS. The existence of transcription factors that bind both DNA and RNA provides an interesting puzzle. Little is known concerning the biological roles of these alternative protein-nucleic acid interactions, and even less is known concerning the structural basis for dual nucleic acid specificity. We discuss how these natural examples have motivated us to identify artificial RNA sequences that competitively inhibit a DNA-binding transcription factor not known to have a natural RNA partner. The identification of such RNAs raises the possibility that RNA binding by DNA-binding proteins is more common than currently appreciated.

Animals↗
PubMed2002

Binding stoichiometry of an RNA aptamer and its transcription factor target.

RNA molecules serve informational, structural, and catalytic roles in cells. RNA also offers an interesting raw material for the design or genetic selection of modifiers of gene expression. We have been interested in the possibility that natural and/or artificial RNA ligands might be identified for DNA-binding proteins. With these concepts in mind, our laboratory previously isolated a 31-nucleotide RNA aptamer that specifically binds to human transcription factor NF-kappaB. This RNA aptamer (alpha-p50) competitively inhibits DNA binding by NF-kappaB in vitro. The aptamer may target the DNA-binding groove formed by the junction of the two monomers of NF-kappaB, perhaps mimicking kappaB duplex DNA. This model predicts a binding stoichiometry of one RNA aptamer per NF-kappaB dimer. To test this hypothesis, two complementary biophysical methods were utilized. Both analytical ultracentrifugation and microelectrospray mass spectrometry suggest that 1 mol of alpha-p50 RNA binds per mole of NF-kappaB p50 homodimer. Such a result is consistent with the observed ability of the RNA aptamer to block the access of transcription factor NF-kappaB to its binding site on DNA and highlights the question of how an RNA stem-loop structurally mimics a DNA duplex. This work also demonstrates the successful application of mass spectrometry to characterize noncovalent RNA/protein interactions.

Protein Binding↗
PubMed2002

Dominant effect of protein charge rather than protein shape in apparent DNA bending by engineered bZIP domains.

We are interested in how asymmetric charge neutralization of DNA by proteins results in DNA bending. We have previously reported electrophoretic phasing experiments utilizing homodimer peptides derived from GCN4, a yeast basic zipper (bZIP) transcription factor. Here we report the results of experiments that examine the importance of peptide sequence context in DNA bending and test the hypothesis that peptide structural asymmetry causes electrophoretic anomalies in the absence of DNA bending. We prepared two new series of bZIP peptides that differed dramatically in overall size, structure, and peptide sequence near the DNA. The magnitude of apparent DNA bending is independent of the structure of the protein. This result reduces the concern that bZIP protein structure causes electrophoretic anomalies in the absence of DNA bending. In all cases, both the magnitude and direction of the apparent DNA bend angle are strongly dependent on the local peptide charge. We attempted to validate independently our results with a minicircle competition binding assay. Binding preferences of severalfold for properly phased circular versus linear DNA templates were predicted. However, no binding preferences were observed. We propose that the minicircle binding assay may be intrinsically insensitive to DNA bending or flexibility induced by the bZIP peptides studied, and we provide a unifying explanation for the discrepancies between the cyclization and electrophoretic experiments.

Amino Acid Sequence↗
PubMed2002

DNA bending by bZIP charge variants: a unified study using electrophoretic phasing and fluorescence resonance energy transfer.

The role of asymmetric charge neutralization as a primary determinant of protein-induced DNA helical bending remains controversial. Electrophoretic phasing experiments have been conducted previously for peptides derived from the yeast basic leucine zipper (bZIP) transcription factor GCN4 bound to AP-1 sites in duplex DNA. Mutations altering the electrostatic character of amino acids close to the DNA backbone result in phase-dependent gel mobility changes, interpreted as evidence of DNA bending. However, alternate interpretations are possible. The effect of electrostatic interactions on DNA conformation has now been investigated further, using purified peptides having indistinguishable AP-1 DNA affinity. Two independent techniques have been employed: electrophoretic phasing and fluorescence resonance energy transfer (FRET). The phasing results imply DNA bending by bZIP charge variants, consistent with earlier findings. FRET studies yield the mean 5' end to 3' end distance of AP-1 DNA when free or bound to neutral or charged bZIP peptides. These distances were reduced in the charged variant complexes relative to those in the free duplex and the wild-type complex. Bending of the DNA helical axis is shown by molecular modeling to be the simplest interpretation of these results. The electrophoretic phasing and FRET results thus offer two mutually supportive lines of evidence for induced bending of the DNA helical axis due to asymmetric changes in charge density caused by the electrostatic character of the amino acids residing near the DNA backbone.

Basic-Leucine Zipper Transcription Factors↗
PubMed2002

Charge neutralization and DNA bending by the Escherichia coli catabolite activator protein.

We are interested in the role of asymmetric phosphate neutralization in DNA bending induced by proteins. We describe an experimental estimate of the actual electrostatic contribution of asymmetric phosphate neutralization to the bending of DNA by the Escherichia coli catabolite activator protein (CAP), a prototypical DNA-bending protein. Following assignment of putative electrostatic interactions between CAP and DNA phosphates based on X-ray crystal structures, appropriate phosphates in the CAP half-site DNA were chemically neutralized by methylphosphonate substitution. DNA shape was then evaluated using a semi-synthetic DNA electrophoretic phasing assay. Our results confirm that the unmodified CAP DNA half-site sequence is intrinsically curved by 26 degrees in the direction enhanced in the complex with protein. In the absence of protein, neutralization of five appropriate phosphates increases DNA curvature to 32 degrees (approximately 23% increase), in the predicted direction. Shifting the placement of the neutralized phosphates changes the DNA shape, suggesting that sequence-directed DNA curvature can be modified by the asymmetry of phosphate neutralization. We suggest that asymmetric phosphate neutralization contributes favorably to DNA bending by CAP, but cannot account for the full DNA deformation.

Consensus Sequence↗
PubMed2002

Functional studies of potential intrastrand triplex elements in the Escherichia coli genome.

We previously used a pattern recognition program for nucleic acids to detect sequences with the potential to form intrastrand triplexes. Potential intrastrand triplex (PIT) element families were found in Escherichia coli, Synechocystis sp. and Haemophilus influenza. We were particularly intrigued with the family found in E. coli, which contained 25 dispersed copies of a particular PIT sequence corresponding to the purine triplex motif. E. coli PIT elements appear to occur exclusively in non-coding regions. We now report biochemical experiments testing the interaction of E. coli PIT elements with polymerases and single-stranded DNA-binding protein (SSB). The elements were also tested in genetic experiments as promoters, transcription terminators, or replication pause sites in E. coli. We show that PIT elements display provocative characteristics in certain biochemical assays. When appropriately oriented, the elements block elongation by Taq DNA polymerase at 72 degrees C, but not elongation by T7 DNA polymerase at 37 degrees C. The G-rich strand of the E. coli PIT sequence folds into a form with reduced affinity for SSB. On the other hand, in vivo studies did not detect replication delays for conjugal transfer of episomes containing PIT elements. These sequences were shown not to act as promoters, but the presence of PIT elements in RNA leaders upstream of a coding region could strongly influence expression of the downstream gene. These effects were shown to be post-transcriptional and were solely dependent on the Watson-Crick stem-loop structure within the PIT element. Thus, although PIT element DNA displays unusual biochemical properties, it remains unknown how these elements arose, and why they persist in the E. coli genome.

Base Sequence↗