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.

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.

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.

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.