Structures of riboswitch RNA reaction states by mix-and-inject XFEL serial crystallography.

Riboswitches are structural RNA elements that are generally located in the 5' untranslated region of messenger RNA. During regulation of gene expression, ligand binding to the aptamer domain of a riboswitch triggers a signal to the downstream expression platform. A complete understanding of the structural basis of this mechanism requires the ability to study structural changes over time. Here we use femtosecond X-ray free electron laser (XFEL) pulses to obtain structural measurements from crystals so small that diffusion of a ligand can be timed to initiate a reaction before diffraction. We demonstrate this approach by determining four structures of the adenine riboswitch aptamer domain during the course of a reaction, involving two unbound apo structures, one ligand-bound intermediate, and the final ligand-bound conformation. These structures support a reaction mechanism model with at least four states and illustrate the structural basis of signal transmission. The three-way junction and the P1 switch helix of the two apo conformers are notably different from those in the ligand-bound conformation. Our time-resolved crystallographic measurements with a 10-second delay captured the structure of an intermediate with changes in the binding pocket that accommodate the ligand. With at least a 10-minute delay, the RNA molecules were fully converted to the ligand-bound state, in which the substantial conformational changes resulted in conversion of the space group. Such notable changes in crystallo highlight the important opportunities that micro- and nanocrystals may offer in these and similar time-resolved diffraction studies. Together, these results demonstrate the potential of 'mix-and-inject' time-resolved serial crystallography to study biochemically important interactions between biomacromolecules and ligands, including those that involve large conformational changes.

Consensus structure of Pf1 filamentous bacteriophage from X-ray fibre diffraction and solid-state NMR.

Filamentous bacteriophages (filamentous bacterial viruses or Inovirus) are simple and well-characterised macromolecular assemblies that are widely used in molecular biology and biophysics, both as paradigms for studying basic biological questions and as practical tools in areas as diverse as immunology and solid-state physics. The strains fd, M13 and f1 are virtually identical filamentous phages that infect bacteria expressing F-pili, and are sometimes grouped as the Ff phages. For historical reasons fd has often been used for structural studies, but M13 and f1 are more often used for biological experiments. Many other strains have been identified that are genetically quite distinct from Ff and yet have a similar molecular structure and life cycle. One of these, Pf1, gives the highest resolution X-ray fibre diffraction patterns known for filamentous bacteriophage. These diffraction patterns have been used in the past to derive a molecular model for the structure of the phage. Solid-state NMR experiments have been used in separate studies to derive a significantly different model of Pf1. Here we combine previously published X-ray fibre diffraction data and solid-state NMR data to give a consensus structure model for Pf1 filamentous bacteriophage, and we discuss the implications of this model for assembly of the phage at the bacterial membrane.

Internal coordinates for molecular dynamics and minimization in structure determination and refinement.

We present a software module which allows one to efficiently perform molecular dynamics and local minimization calculations in internal coordinates when incorporated into a molecular dynamics package. We have implemented a reference interface to the NIH version of the X-PLOR structure refinement package and we show that the module provides superior torsion-angle dynamics functionality relative to the native X-PLOR implementation. The module has been designed in a portable fashion so that interfacing it with other packages should be relatively easy. Other features of the module include the ability to define rather general internal coordinates, an accurate integration algorithm which can automatically adjust the integration step size, and a modular design, which facilitates extending and enhancing the module.

The VMD-XPLOR visualization package for NMR structure refinement.

In this paper we present the VMD-XPLOR package combining the XPLOR refinement program and the VMD visualization program and including extensions for use in the determination of biomolecular structures from NMR data. The package allows one to pass structures to and to control VMD from the XPLOR scripting level. The VMD graphical interface has been customized for NMR structure refinement, including support to manipulate coordinates interactively while graphically visualizing NMR experimental information in the context of a molecular structure. Finally, the VMD-XPLOR interface is modular so that it is readily transferable to other refinement programs (such as CNS).