Dynamic RNA molecules carry out essential processes in the cell including translation and splicing. Base-pair interactions stabilize RNA into relatively rigid structures, while flexible non-base-paired regions allow RNA to undergo conformational changes required for function. To advance our understanding of RNA folding and dynamics it is critical to know the flexibility of these un-base-paired regions and how it depends on counterions. Yet, information about nucleic acid polymer properties is mainly derived from studies of ssDNA. Here we measure the persistence lengths (l p ) of ssRNA. We observe valence and ionic strength-dependent differences in l p in a direct comparison between 40-mers of deoxythymidylate (dT 40 ) and uridylate (rU 40 ) measured using the powerful combination of SAXS and smFRET. We also show that nucleic acid flexibility is influenced by local environment (an adjoining double helix). Our results illustrate the complex interplay between conformation and ion environment that modulates nucleic acid function in vivo.single molecule FRET | small angle X-ray scattering | worm-like chain | ion-nucleic acid interactions N ucleic acids in the cell are dynamic and undergo structural changes as they transmit and process genetic information. Dynamic processes related to biological function (e.g., transcription for DNA and recognition and folding for RNA) involve nonbase-paired regions that confer flexibility to the overall structure. For RNAs like riboswitches that exchange between multiple structures in equilibrium (1), conformational disorder is often an intrinsic property of the molecule and important for biological function. Even relatively stable molecules like catalytic introns and transfer RNA must pass through a disordered phase while folding. Thus, progress toward a mechanistic understanding of RNA folding and dynamics will require detailed knowledge of nucleic acid chain flexibility and its dependence on base content, solution conditions, and molecular context.In light of its importance to biology, it is surprising that RNA flexibility has not been studied in as much detail as DNA flexibility. Despite the chemical similarity of the RNA and DNA backbone, there is ample evidence from X-ray crystallography that the identity of the sugar (ribose vs. deoxy-ribose) affects backbone conformations (2). However, researchers have used the properties of DNA to understand RNA folding (3) because corresponding information for RNA was lacking. This difficulty motivates our present efforts to measure and directly compare the flexibilities of single-stranded nucleic acids (ssRNA and ssDNA).In the cell, RNA and DNA interact with cations that screen the negatively charged phosphate backbone. Both diffuse and specifically bound ions are important for RNA folding (4), and divalent ions are almost always required to stabilize RNA tertiary structures (5-8). A full mechanistic description of these ion effects is complicated by the fact that ions can interact with RNA differently during various stages of folding (9, ...

Mammalian phenylalanine hydroxylase (PheH) is an allosteric enzyme that catalyzes the first step in the catabolism of the amino acid phenylalanine. Following allosteric activation by high phenylalanine levels, the enzyme catalyzes the pterin-dependent conversion of phenylalanine to tyrosine. Inability to control elevated phenylalanine levels in the blood leads to increased risk of mental disabilities commonly associated with the inherited metabolic disorder, phenylketonuria. Although extensively studied, structural changes associated with allosteric activation in mammalian PheH have been elusive. Here, we examine the complex allosteric mechanisms of rat PheH using X-ray crystallography, isothermal titration calorimetry (ITC), and small-angle X-ray scattering (SAXS). We describe crystal structures of the pre-activated state of the PheH tetramer depicting the regulatory domains docked against the catalytic domains and preventing substrate binding. Using SAXS, we further describe the domain movements involved in allosteric activation of PheH in solution and present the first demonstration of chromatography-coupled SAXS with Evolving Factor Analysis (EFA), a powerful method for separating scattering components in a model-independent way. Together, these results support a model for allostery in PheH in which phenylalanine stabilizes the dimerization of the regulatory domains and exposes the active site for substrate binding and other structural changes needed for activity.

The results of more than a dozen single-molecule Förster Resonance Energy Transfer (smFRET) experiments suggest that chemically unfolded polypeptides invariably collapse from an expanded random coil to more compact dimensions as the denaturant concentration is reduced. In sharp contrast, small angle x-ray scattering (SAXS) studies suggest that, at least for single domain proteins at non-zero denaturant concentrations, such compaction may be rare. Here we explore this discrepancy by studying protein L, a protein previously studied by SAXS (at 5°C), which suggested fixed unfolded-state dimensions from 1.4 to 5 M guanidine hydrochloride (GuHCl), and by smFRET (at 25°C), which suggested that, in contrast, the chain contracts by 15-30% over this same denaturant range. Repeating the earlier SAXS study under the same conditions employed in the smFRET studies we observe little if any evidence that the unfolded state of protein L contracts as the concentration of GuHCl is reduced. For example, scattering profiles (and thus the shape and dimensions) collected within ~4 ms after dilution to as low as 0.67 M GuHCl are effectively indistinguishable from those observed at equilibrium at higher denaturant. Our results thus argue that the disagreement between SAXS and smFRET is statistically significant, and that the experimental evidence in favor of obligate polypeptide collapse at low denaturant cannot yet be considered conclusive.

Protein dynamics are integral to biological function, yet few techniques are sensitive to collective atomic motions. A long-standing goal of X-ray crystallography has been to combine structural information from Bragg diffraction with dynamic information contained in the diffuse scattering background. However, the origin of macromolecular diffuse scattering has been poorly understood, limiting its applicability. We present a finely sampled diffuse scattering map from triclinic lysozyme with unprecedented accuracy and detail, clearly resolving both the inter-and intramolecular correlations. These correlations are studied theoretically using both all-atom molecular dynamics and simple vibrational models. Although lattice dynamics reproduce most of the diffuse pattern, protein internal dynamics, which include hinge-bending motions, are needed to explain the short-ranged correlations revealed by Patterson analysis. These insights lay the groundwork for animating crystal structures with biochemically relevant motions.