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Background: Post-transcriptional modifications of rRNAs play important roles in biogenesis and function of ribosome. Results: NAT10 is an ATP-dependent RNA acetyltransferase responsible for N 4 -acetylcytidine formation of 18 S rRNA. Conclusion: NAT10 and ac 4 C1842 are required for pre-18 S rRNA processing. Significance: 40 S subunit formation is regulated by a single acetylation of 18 S rRNA, implying a regulatory mechanism for ribosome biogenesis by sensing the cellular energy budget.

In late 2022, SARS-CoV-2 Omicron subvariants have become highly diversified, and XBB is spreading rapidly around the world. Our phylogenetic analyses suggested that XBB emerged through the recombination of two cocirculating BA.2 lineages, BJ.1 and BM.1.1.1 (a progeny of BA.2.75), during the summer of 2022. XBB.1 is the variant most profoundly resistant to BA.2/5 breakthrough infection sera to date and is more fusogenic than BA.2.75. The recombination breakpoint is located in the receptor-binding domain of spike, and each region of the recombinant spike confers immune evasion and increases fusogenicity. We further provide the structural basis for the interaction between XBB.1 spike and human ACE2. Finally, the intrinsic pathogenicity of XBB.1 in male hamsters is comparable to or even lower than that of BA.2.75. Our multiscale investigation provides evidence suggesting that XBB is the first observed SARS-CoV-2 variant to increase its fitness through recombination rather than substitutions.

Antiviral treatments targeting the coronavirus disease 2019 are urgently required. We screened a panel of already-approved drugs in a cell culture model of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) and identified two new agents having higher antiviral potentials than the drug candidates such as Remdesivir and Chroloquine in VeroE6/TMPRSS2 cells: the anti-inflammatory drug Cepharanthine and HIV protease inhibitor Nelfinavir. Cepharanthine inhibited SARS-CoV-2 entry through the blocking of viral binding to target cells, whilst Nelfinavir suppressed viral replication partly by protease inhibition. Consistent with their different modes of action, synergistic effect of this combined treatment to limit SARS-CoV-2 proliferation was highlighted. Mathematical modeling in vitro antiviral activity coupled with the calculated total drug concentrations in the lung predicts that Nelfinavir will shorten the period until viral clearance by 4.9-days and the combining Cepharanthine/Nelfinavir enhanced their predicted efficacy. These results warrant further evaluation of the potential anti-SARS-CoV-2 activity of Cepharanthine and Nelfinavir.

Pyrrolysyl-tRNA synthetase (PylRS) is a major tool in genetic code expansion with non-canonical amino acids, yet understanding of its structure and activity is incomplete. Here we describe the crystal structure of the previously uncharacterized essential N-terminal domain of this unique enzyme in complex with tRNAPyl. This structure explains why PylRS remains orthogonal in a broad range of organisms, from bacteria to humans. The structure also illustrates why tRNAPyl recognition by PylRS is anticodon-independent; the anticodon does not contact the enzyme. Using standard microbiological culture equipment, we then established a new method for laboratory evolution – a non-continuous counterpart of the previously developed phage-assisted continuous evolution. With this method, we evolved novel PylRS variants with enhanced activity and amino acid specificity. We finally employed an evolved PylRS variant to determine its N-terminal domain structure and show how its mutations improve PylRS activity in the genetic encoding of a non-canonical amino acid.