Structural studies put phage defense mystery on the RADAR.

Phage restriction by adenosine deaminase acting on RNA (RADAR) is a process by which bacteria may alter their own transcriptome to resist bacteriophage. In this issue of Cell, Duncan-Lowey and Tal et al. and Gao et al. both show RADAR proteins assemble into massive molecular complexes but present distinct views about how these assemblies obstruct phage.

Recent advances in the structural biology of encapsulin bacterial nanocompartments.

Large capsid-like nanocompartments called encapsulins are common in bacteria and archaea and contain cargo proteins with diverse functions. Advances in cryo-electron microscopy have enabled structure determination of many encapsulins in recent years. Here we summarize findings from recent encapsulin structures that have significant implications for their biological roles. We also compare important features such as the E-loop, cargo-peptide binding site, and the fivefold axis channel in different structures. In addition, we describe the discovery of a flavin-binding pocket within the encapsulin shell that may reveal a role for this nanocompartment in iron metabolism.

Cryo-EM structure of a thermostable bacterial nanocompartment.

Protein nanocompartments are widespread in bacteria and archaea, but their functions are not yet well understood. Here, the cryo-EM structure of a nanocompartment from the thermophilic bacterium Thermotoga maritima is reported at 2.0 Šresolution. The high resolution of this structure shows that interactions in the E-loop domain may be important for the thermostability of the nanocompartment assembly. Also, the channels at the fivefold axis, threefold axis and dimer interface are assessed for their ability to transport iron. Finally, an unexpected flavin ligand was identified on the exterior of the shell, indicating that this nanocompartment may also play a direct role in iron metabolism.

SHAPE Profiling to Probe Group II Intron Conformational Dynamics During Splicing.

Selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) is a widely used technique for studying the structure and function of RNA molecules. It characterizes the flexibility of single nucleotides in the context of the local RNA structure. Here we describe the application of SHAPE-MaP (mutational profiling) to study different conformational states of the group II intron during the self-splicing reaction.

Determinants of adenine-mutagenesis in diversity-generating retroelements.

Diversity-generating retroelements (DGRs) vary protein sequences to the greatest extent known in the natural world. These elements are encoded by constituents of the human microbiome and the microbial 'dark matter'. Variation occurs through adenine-mutagenesis, in which genetic information in RNA is reverse transcribed faithfully to cDNA for all template bases but adenine. We investigated the determinants of adenine-mutagenesis in the prototypical Bordetella bacteriophage DGR through an in vitro system composed of the reverse transcriptase bRT, Avd protein, and a specific RNA. We found that the catalytic efficiency for correct incorporation during reverse transcription by the bRT-Avd complex was strikingly low for all template bases, with the lowest occurring for adenine. Misincorporation across a template adenine was only somewhat lower in efficiency than correct incorporation. We found that the C6, but not the N1 or C2, purine substituent was a key determinant of adenine-mutagenesis. bRT-Avd was insensitive to the C6 amine of adenine but recognized the C6 carbonyl of guanine. We also identified two bRT amino acids predicted to nonspecifically contact incoming dNTPs, R74 and I181, as promoters of adenine-mutagenesis. Our results suggest that the overall low catalytic efficiency of bRT-Avd is intimately tied to its ability to carry out adenine-mutagenesis.

Structural basis for the second step of group II intron splicing.

The group II intron and the spliceosome share a common active site architecture and are thought to be evolutionarily related. Here we report the 3.7 Å crystal structure of a eukaryotic group II intron in the lariat-3' exon form, immediately preceding the second step of splicing, analogous to the spliceosomal P complex. This structure reveals the location of the intact 3' splice site within the catalytic core of the group II intron. The 3'-OH of the 5' exon is positioned in close proximity to the 3' splice site for nucleophilic attack and exon ligation. The active site undergoes conformational rearrangements with the catalytic triplex having different configurations before and after the second step of splicing. We describe a complete model for the second step of group II intron splicing that incorporates a dynamic catalytic triplex being responsible for creating the binding pocket for 3' splice site capture.

Structure determination of group II introns.

Group II introns are self-splicing catalytic RNAs that are able to excise themselves from pre-mRNAs using a mechanism identical to that utilized by the spliceosome. Both structural and phylogenetic data support the hypothesis that group II introns and the spliceosome share a common ancestor. Structures of group II introns have given insight into the active site required for the catalysis of RNA splicing. This review outlines crucial aspects of the structure determination of group II introns such as sample preparation and data processing. Given that group II introns are large RNAs that must be synthesized through in vitro transcription, there are special considerations that must be taken into account in terms of purification and crystallization, as compared to the isolation of large intact ribonucleoprotein complexes such as the ribosome. We specifically focus on the methodology used to determine the structure of the eukaryotic group II intron lariat from the brown algae Pylaiella littoralis. The techniques described in this review can also be applied for the structure determination of other large RNAs.