Pyrrole-imidazole Polyamides Research Articles

Inhibition of RNA polymerase II transcription in human cells by synthetic DNA-binding ligands.

Sequence-specific DNA-binding small molecules that can permeate human cells potentially could regulate transcription of specific genes. Multiple cellular DNA-binding transcription factors are required by HIV type 1 for RNA synthesis. Two pyrrole-imidazole polyamides were designed to bind DNA sequences immediately adjacent to binding sites for the transcription factors Ets-1, lymphoid-enhancer binding factor 1, and TATA-box binding protein. These synthetic ligands specifically inhibit DNA-binding of each transcription factor and HIV type 1 transcription in cell-free assays. When used in combination, the polyamides inhibit virus replication by >99% in isolated human peripheral blood lymphocytes, with no detectable cell toxicity. The ability of small molecules to target predetermined DNA sequences located within RNA polymerase II promoters suggests a general approach for regulation of gene expression, as well as a mechanism for the inhibition of viral replication.

Cooperative Hairpin Dimers for Recognition of DNA by Pyrrole–Imidazole Polyamides

Cooperative Hairpin Dimers for Recognition of DNA by Pyrrole–Imidazole Polyamides

Cooperative Hairpin Dimers for Recognition of DNA by Pyrrole–Imidazole Polyamides

Cooperative Hairpin Dimers for Recognition of DNA by Pyrrole–Imidazole Polyamides

Cooperative Hairpin Dimers for Recognition of DNA by Pyrrole-Imidazole Polyamides.

A doubling of the length of binding site for the same size of ligand is achieved by the title compound by formation of a cooperative hairpin dimer on binding to DNA (depicted schematically below). The binding affinity and selectivity are unaffected by this new binding pattern. Circles represent heterocyclic rings, and diamonds and curved lines represent β-alanine and (R)-2,4-diaminobutyric acid residues, respectively.

Inhibition of major-groove-binding proteins by pyrrole-imidazole polyamides with an Arg-Pro-Arg positive patch

Background: Gene-specific targeting of any protein-DNA complex by small molecules is a challenging goal at the interface of chemistry and biology. Polyamides containing N-methylimidazole and N-methylpyrrole amino acids are synthetic ligands that have an affinity and specificity for DNA comparable to many naturally occurring DNA-binding proteins. It has been shown that an eight-ring hairpin polyamide targeted to a specific minor-groove contact within a transcription factor binding site can inhibit protein-DNA binding and gene transcription. Polyamides and certain major-groove-binding proteins have been found to co-occupy the DNA helix, however. To expand the number of genes that can be targeted by pyrrole/imidazole polyamides, we set out to develop a class of polyamides that can selectively inhibit major-groove-binding proteins. Results: An eight-ring hairpin polyamide conjugated to a carboxy-terminal Arg-Pro-Arg tripeptide was designed to deliver a positive residue to the DNA backbone and interfere with protein-phosphate contacts. Gel mobility shift analysis demonstrated that a polyamide hairpin-Arg-Pro-Arg binding in the minor groove selectively inhibits binding of the transcription factor GCN4 (222–281) in the adjacent major groove. Substitution within the Arg-Pro-Arg revealed that each residue was required for optimal GCN4 inhibition. Conclusions: A pyrrole-imidazole polyamide that binds to a predetermined site in the DNA minor groove and delivers a positive patch to the DNA backbone can selectively inhibit a DNA-binding protein that recognizes the adjacent major groove. A subtle alteration of the DNA microenvironment targeted to a precise location within a specific DNA sequence could achieve both gene-specific and protein-specific targeting.

Structural basis for G.C recognition in the DNA minor groove.

Small molecules that target specific DNA sequences offer a potentially general approach for the regulation of gene expression. Pyrrole-imidazole polyamides represent the only class of synthetic small molecules that can bind predetermined DNA sequences with affinities and specificities comparable to DNA binding proteins. Antiparallel side-by-side pairings of two aromatic amino acids, imidazole (Im) and pyrrole (Py), distinguish G.C from C.G, and both from A.T/T.A base pairs. A high resolution X-ray crystal structure of a four-ring pyrrole-imidazole polyamide specifically bound as a dimer to a six-base pair predetermined DNA site reveals a structural framework of hydrogen bonds and interactions with the walls of the minor groove that underlies the pairing rules for DNA recognition.

Polyamides as Artificial Regulators of Gene Expression

Interference with the transcription of specific genes in vivo can be achieved with artificial, low molecular weight pyrrole-imidazole polyamides, as shown by recent studies by Gottesfeld, Dervan et al. This is possible because these ligands are capable of binding with high affinity and specificity to predetermined DNA sequences in the minor groove of double-stranded DNA (see schematic representation on the right; gray: pyrrole, black: imidazole units).

On the pairing rules for recognition in the minor groove of DNA by pyrrole-imidazole polyamides.

Cell-permeable small molecules that target predetermined DNA sequences with high affinity and specificity have the potential to control gene expression. A binary code has been developed to correlate DNA sequence with side-by-side pairings between N-methylpyrrole (Py) and N-methylimidazole (Im) carboxamides in the DNA minor groove. We set out to determine the relative energetics of pairings of Im/Py, Py/Im, Im/Im, and Py/Py for targeting G.C and A.T base pairs. A key specificity issue, which has not been previously addressed, is whether an Im/Im pair is energetically equivalent to an Im/Py pair for targeting G.C base pairs. Equilibrium association constants were determined at two five-base-pair sites for a series of four six-ring hairpin polyamides, in order to test the relative energetics of the four aromatic amino-acid pairings opposite G.C and A.T base pairs in the central position. We observed that a G.C base pair was effectively targeted with Im/Py but not Py/Im, Py/Py, or Im/Im. The A.T base pair was effectively targeted with Py/Py but not Im/Py, Py/Im, or Im/Im. An Im/Im pairing is energetically disfavored for the recognition of both A.T and G.C. This specificity will create important limitations on undesirable slipped motifs that are available for unlinked dimers in the minor groove. Baseline energetic parameters will thus be created which, using the predictability of the current pairing rules for specific molecular recognition of double-helical DNA, will guide further second-generation polyamide design for DNA recognition.

Discrimination of 5‘-GGGG-3‘, 5‘-GCGC-3‘, and 5‘-GGCC-3‘ Sequences in the Minor Groove of DNA by Eight-Ring Hairpin Polyamides

Eight-ring hairpin polyamides which differ only by the linear arrangement of pyrrole (Py) and imidazole (Im) amino acids were designed for recognition of six base pair DNA sequences containing four contiguous G,C base pairs. The respective DNA binding properties of three polyamides, ImImPyPy-γ-ImImPyPy-β-Dp, ImPyImPy-γ-ImPyImPy-β-Dp, and ImImImIm-γ-PyPyPyPy-β-Dp, were analyzed by footprinting and affinity cleavage on a DNA fragment containing the respective match sites 5‘-TGGCCA-3‘, 5‘-TGCGCA-3‘, and 5‘-TGGGGA-3‘. Quantitative footprint titrations demonstrate that ImImPyPy-γ-ImImPyPy-β-Dp binds the designed match site 5‘-TGGCCA-3‘ with an equilibrium association constant of Ka = 1 × 1010 M-1 and >250-fold specificity versus the mismatch sequences, 5‘-TGCGCA-3‘ and 5‘-TGGGGA-3‘. The polyamides ImPyImPy-γ-ImPyImPy-β-Dp and ImImImIm-γ-PyPyPyPy-β-Dp recognize their respective 5‘-TGCGCA-3‘ and 5‘-TGGGGA-3‘ match sites with reduced affinity relative to ImImPyPy-γ-ImImPyPy-β-Dp, but again with high specificity with regard to mismatch sites. These results expand the DNA sequence repertoire targeted by pyrrole-imidazole polyamides and identify sequence composition effects which will guide further second-generation polyamide design for DNA recognition.

Targeting the minor groove of DNA

Small molecules that specifically bind with high affinity to any predetermined DNA sequence in the human genome will be useful tools in molecular biology and, potentially, in human medicine. Pairing rules have been developed to control rationally the sequence specifity of minor groove binding polyamides containing N-methylimidazole and N-methylpyrrole amino acids. Using simple molecular shapes and a two-letter aromatic amino acid code, pyrrole-imidazole polyamides achieve affinities and specificities comparable to DNA-binding proteins.

Regulation of gene expression by small molecules.

Small molecules that target specific DNA sequences have the potential to control gene expression. Ligands designed for therapeutic application must bind any predetermined DNA sequence with high affinity and permeate living cells. Synthetic polyamides containing N-methylimidazole and N-methylpyrrole amino acids have an affinity and specificity for DNA comparable to naturally occurring DNA-binding proteins. We report here that an eight-ring polyamide targeted to a specific region of the transcription factor TFIIIA binding site interferes with 5S RNA gene expression in Xenopus kidney cells. Our results indicate that pyrrole-imidazole polyamides are cell-permeable and can inhibit the transcription of specific genes.

Binding of a hairpin polyamide in the minor groove of DNA: sequence-specific enthalpic discrimination.

Hairpin polyamides are synthetic ligands for sequence-specific recognition in the minor groove of double-helical DNA. A thermodynamic characterization of the DNA-binding properties exhibited by a six-ring hairpin polyamide, ImPyPy-gamma-PyPyPy-beta-Dp (where Im = imidazole, Py = pyrrole, gamma = gamma-aminobutyric acid, beta = beta-alanine, and Dp = dimethylaminopropylamide), reveals an approximately 1-2 kcal/mol greater affinity for the designated match site, 5'-TGTTA-3', relative to the single base pair mismatch sites, 5'-TGGTA-3' and 5'-TATTA-3'. The enthalpy and entropy data at 20 degrees C reveal this sequence specificity to be entirely enthalpic in origin. Correlations between the thermodynamic driving forces underlying the sequence specificity exhibited by ImPyPy-gamma-PyPyPy-beta-Dp and the structural properties of the heterodimeric complex of PyPyPy and ImPyPy bound to the minor groove of DNA provide insight into the molecular forces that govern the affinity and specificity of pyrrole-imidazole polyamides.

Recognition of DNA by designed ligands at subnanomolar concentrations.

Small molecules that specifically bind with high affinity to any predetermined DNA sequence in the human genome would be useful tools in molecular biology and potentially in human medicine. Simple rules have been developed to control rationally the sequence specificity of minor-groove-binding polyamides containing N-methylimidazole and N-methylpyrrole amino acids. Two eight-ring pyrrole-imidazole polyamides differing in sequence by a single amino acid bind specifically to respective six-base-pair target sites which differ in sequence by a single base pair. Binding is observed at subnanomolar concentrations of ligand. The replacement of a single nitrogen atom with a C-H regulates affinity and specificity by two orders of magnitude. The broad range of sequences that can be specifically targeted with pyrrole-imidazole polyamides, coupled with an efficient solid-phase synthesis methodology, identify a powerful class of small molecules for sequence-specific recognition of double-helical DNA.