Structure determination of gold nanoparticles (AuNPs) is necessary for understanding their physical and chemical properties, and only one AuNP larger than 1 nm in diameter, an Au102NP, has been solved to atomic resolution. Whereas the Au102NP structure was determined by X-ray crystallography, other large AuNPs have proved refractory to this approach. Here we report the structure determination of an Au68NP at atomic resolution by aberration-corrected transmission electron microscopy (AC-TEM), performed with the use of a minimal electron dose, an approach that should prove applicable to metal NPs in general. The structure of the Au68NP was supported by small angle X-ray scattering (SAXS) and by comparison of observed infrared (IR) absorption spectra with calculations by density functional theory (DFT).

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Summary The protein density and arrangement of subunits of a complete, 31-protein, RNA polymerase II (pol II) transcription pre-initiation complex (PIC) were determined by cryo-electron microscopy and a combination of chemical cross-linking and mass spectrometry. The PIC showed a marked division in two parts, one containing all the general transcription factors (GTFs), and the other pol II. Promoter DNA was associated only with the GTFs, suspended above the pol II cleft and not in contact with pol II. This structural principle of the PIC underlies its conversion to a transcriptionally active state; the PIC is poised for the formation of a transcription bubble and descent of the DNA into the pol II cleft.

Splicing of pre-mRNA occurs in a multicomponent macromolecular machine--the spliceosome. The spliceosome can be assembled in vitro by a stepwise assembly of a number of snRNPs and additional proteins on exogenously added pre-mRNA. In contrast, splicing in vivo occurs in preformed particles where endogenous pre-mRNAs are packaged with all five spliceosomal U snRNPs (penta-snRNP) together with other splicing factors. Here we present a three-dimensional image reconstruction by cryo-electron microscopy of native spliceosomes, derived from cell nuclei, at a resolution of 20 angstroms. The structure revealed an elongated globular particle made up of two distinct subunits connected to each other leaving a tunnel in between. We show here that the larger subunit is a suitable candidate to accommodate the penta-snRNP, and that the tunnel could accommodate the pre-mRNA component of the spliceosome. The features this structure reveals provide new insight into the global architecture of the native splicing machine.

Most eukaryotic pre-mRNAs contain non-coding sequences (introns) that must be removed in order to accurately place the coding sequences (exons) in the correct reading frame. This critical regulatory pre-mRNA splicing event is fundamental in development and cancer. It occurs within a mega-Dalton multicomponent machine composed of RNA and proteins, which undergoes dynamic changes in RNA-RNA, RNA-protein, and protein-protein interactions during the splicing reaction. Recent years have seen progress in functional and structural analyses of the splicing machine and its subcomponents, and this review is focused on structural aspects of the pre-mRNA splicing machine and their mechanistic implications on the splicing of multi-intronic pre-mRNAs. It brings together, in a comparative manner, structural information on spliceosomes and their intermediates in the stepwise assembly process in vitro, and on the preformed supraspliceosomes, which are isolated from living cell nuclei, with a view of portraying a consistent picture.