Examining the capacity of human U1 snRNA variants to facilitate pre-mRNA splicing.

The human U1 snRNA is encoded by a multigene family consisting of transcribed variants and defective pseudogenes. Many variant U1 (vU1) snRNAs have been demonstrated to not only be transcribed but also processed by the addition of a trimethylated guanosine cap, packaged into snRNPs, and assembled into spliceosomes; however, their capacity to facilitate pre-mRNA splicing has, so far, not been tested. A recent systematic analysis of the human snRNA genes identified 178 U1 snRNA genes that are present in the genome as either tandem arrays or single genes on multiple chromosomes. Of these, 15 were found to be expressed in human tissues and cell lines, although at significantly low levels from their endogenous loci, <0.001% of the canonical U1 snRNA. In this study, we found that placing the variants in the context of the regulatory elements of the RNU1-1 gene improves the expression of many variants to levels comparable to the canonical U1 snRNA. Application of a previously established HeLa cell-based minigene reporter assay to examine the capacity of the vU1 snRNAs to support pre-mRNA splicing revealed that even though the exogenously expressed variant snRNAs were enriched in the nucleus, only a few had a measurable effect on splicing.

Atomic Scale Characterization of Fluxional Cation Behavior on Nanoparticle Surfaces: Probing Oxygen Vacancy Creation/Annihilation at Surface Sites.

Oxygen vacancy creation and annihilation are key processes in nonstoichiometric oxides such as CeO2. The oxygen vacancy creation and annihilation rates on an oxide's surface partly govern its ability to exchange oxygen with the ambient environment, which is critical for a number of applications including energy technologies, environmental pollutant remediation, and chemical synthesis. Experimental methods to probe and correlate local oxygen vacancy reaction rates with atomic-level structural heterogeneities would provide significant information for the rational design and control of surface functionality; however, such methods have been unavailable to date. Here, we characterize picoscale fluxional behavior in cations using time-resolved in situ aberration-corrected transmission electron microscopy to locate atomic-level variations in oxygen vacancy creation and annihilation rates on oxide nanoparticle surfaces. Low coordination number sites such as steps and edges, as well as locally strained sites, exhibited the greatest number of cation displacements, implying enhanced surface oxygen vacancy activity at these sites. The approach has potential applications to a much wider class of materials and catalysis problems involving surface and interfacial transport functionalities.

Tracking the picoscale spatial motion of atomic columns during dynamic structural change.

In many materials systems, such as catalytic nanoparticles, the ability to characterize dynamic atomic structural changes is important for developing a more fundamental understanding of functionality. Recent developments in direct electron detection now allow image series to be acquired at frame rates on the order of 1000 frames per second in bright-field transmission electron microscopy (BF TEM), which could potentially allow dynamic changes in the atomic structure of individual nanoparticles to be characterized with millisecond temporal resolution in favorable cases. However, extracting such data from TEM image series requires the development of computational methods that can be applied to very large datasets and are robust in the presence of noise and in the non-ideal imaging conditions of some types of environmental TEM experiments. Here, we present a two-dimensional Gaussian fitting algorithm to track the position and intensities of atomic columns in temporally resolved BF TEM image series. We have tested our algorithm on experimental image series of Ce atomic columns near the surface of a ceria (CeO2) nanoparticle with electron beam doses of ~125-5000 e-Å-2 per frame. The accuracy of the algorithm for locating atomic column positions is compared to that of the more traditional centroid fitting technique, and the accuracy of intensity measurements is evaluated as a function of dose per frame. The code developed here, and the methodology used to explore the errors and limitations of the measurements, could be applied more broadly to any temporally resolved TEM image series to track dynamic atomic column motion.

Approaches to Exploring Spatio-Temporal Surface Dynamics in Nanoparticles with In Situ Transmission Electron Microscopy.

Many nanoparticles in fields such as heterogeneous catalysis undergo surface structural fluctuations during chemical reactions, which may control functionality. These dynamic structural changes may be ideally investigated with time-resolved in situ electron microscopy. We have explored approaches for extracting quantitative information from large time-resolved image data sets with a low signal to noise recorded with a direct electron detector on an aberration-corrected transmission electron microscope. We focus on quantitatively characterizing beam-induced dynamic structural rearrangements taking place on the surface of CeO2 (ceria). A 2D Gaussian fitting procedure is employed to determine the position and occupancy of each atomic column in the nanoparticle with a temporal resolution of 2.5 ms and a spatial precision of 0.25 Å. Local rapid lattice expansions/contractions and atomic migration were revealed to occur on the (100) surface, whereas (111) surfaces were relatively stable throughout the experiment. The application of this methodology to other materials will provide new insights into the behavior of nanoparticle surface reconstructions that were previously inaccessible using other methods, which will have important consequences for the understanding of dynamic structure-property relationships.