Chemical modifications of RNA have essential roles in a vast range of cellular processes. N(6)-methyladenosine (m(6)A) is an abundant internal modification in messenger RNA and long non-coding RNA that can be dynamically added and removed by RNA methyltransferases (MTases) and demethylases, respectively. An MTase complex comprising methyltransferase-like 3 (METTL3) and methyltransferase-like 14 (METTL14) efficiently catalyses methyl group transfer. In contrast to the well-studied DNA MTase, the exact roles of these two RNA MTases in the complex remain to be elucidated. Here we report the crystal structures of the METTL3-METTL14 heterodimer with MTase domains in the ligand-free, S-adenosyl methionine (AdoMet)-bound and S-adenosyl homocysteine (AdoHcy)-bound states, with resolutions of 1.9, 1.71 and 1.61 Å, respectively. Both METTL3 and METTL14 adopt a class I MTase fold and they interact with each other via an extensive hydrogen bonding network, generating a positively charged groove. Notably, AdoMet was observed in only the METTL3 pocket and not in METTL14. Combined with biochemical analysis, these results suggest that in the m(6)A MTase complex, METTL3 primarily functions as the catalytic core, while METTL14 serves as an RNA-binding platform, reminiscent of the target recognition domain of DNA N(6)-adenine MTase. This structural information provides an important framework for the functional investigation of m(6)A.

Another kind of dynamics: Ubiquitin noncovalently dimerizes with a dissociation constant of approximately 5 mM. The two subunits adopt an array of relative orientations, utilizing an interface also for binding to other proteins (see picture). Quaternary fluctuation among members of the dimer ensemble constitutes a different kind of dynamics that complements the tertiary dynamics of each ubiquitin subunit.

Nanoscale correlation of structural information acquisition with specific-molecule identification provides new insight for studying rare subcellular events. To achieve this correlation, scanning electron microscopy has been combined with super-resolution fluorescent microscopy, despite its destructivity when acquiring biological structure information. Here we propose time-efficient non-invasive microsphere-based scanning superlens microscopy that enables the large-area observation of live-cell morphology or sub-membrane structures with sub-diffraction-limited resolution and is demonstrated by observing biological and non-biological objects. This microscopy operates in both non-invasive and contact modes with ∼200 times the acquisition efficiency of atomic force microscopy, which is achieved by replacing the point of an atomic force microscope tip with an imaging area of microspheres and stitching the areas recorded during scanning, enabling sub-diffraction-limited resolution. Our method marks a possible path to non-invasive cell imaging and simultaneous tracking of specific molecules with nanoscale resolution, facilitating the study of subcellular events over a total cell period.

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A bacteriophage can replicate and release virions from a host cell in the lytic cycle or switch to a lysogenic process in which the phage integrates itself into the host genome as a prophage. In Bacillus cells, some types of phages employ the arbitrium communication system, which contains an arbitrium hexapeptide, the cellular receptor AimR and the lysogenic negative regulator AimX. This system controls the decision between the lytic and lysogenic cycles. However, both the mechanism of molecular recognition between the arbitrium peptide and AimR and how downstream gene expression is regulated remain unknown. Here, we report crystal structures for AimR from the SPbeta phage in the apo form and the arbitrium peptide-bound form at 2.20 Å and 1.92 Å, respectively. With or without the peptide, AimR dimerizes through the C-terminal capping helix. AimR assembles a superhelical fold and accommodates the peptide encircled by its tetratricopeptide repeats, which is reminiscent of RRNPP family members from the quorum-sensing system. In the absence of the arbitrium peptide, AimR targets the upstream sequence of the aimX gene; its DNA binding activity is prevented following peptide binding. In summary, our findings provide a structural basis for peptide recognition in the phage lysis-lysogeny decision communication system.

We discuss further observational support of an idea formulated a decade ago by Abramowicz, Kluźniak, McClintock and Remillard. They demonstrated that the 3:2 pairs of frequencies of the twin-peak black hole (BH) high-frequency quasi-periodic oscillations (QPOs) scale inversely with the BH masses and that the scaling covers the entire range from stellar to supermassive BHs. For this reason, they believed that the QPOs may be used for accurate measurements of masses and spins of BHs.

Barrels that build barrels Cells produce specialized machinery to produce functional β-barrel proteins that are involved in cross-membrane transport and membrane protein biogenesis in eukaryotic organelles and some bacteria. Wang et al . studied one of these systems that is itself a β-barrel, the mitochondrial sorting and assembly machinery (SAM), in complex with one of its client proteins, translocase of the outer membrane (TOM). TOM subunits can be added one at a time while the protein is bound to SAM. Compared with the full TOM core complex, the final TOM core subunit clashes with SAM and suggests that release of TOM may be driven by its association. —MAF