Lysosomal endonuclease RNase T2 and PLD exonucleases cooperatively generate RNA ligands for TLR7 activation.

Toll-like receptor 7 (TLR7) is essential for recognition of RNA viruses and initiation of antiviral immunity. TLR7 contains two ligand-binding pockets that recognize different RNA degradation products: pocket 1 recognizes guanosine, while pocket 2 coordinates pyrimidine-rich RNA fragments. We found that the endonuclease RNase T2, along with 5' exonucleases PLD3 and PLD4, collaboratively generate the ligands for TLR7. Specifically, RNase T2 generated guanosine 2',3'-cyclic monophosphate-terminated RNA fragments. PLD exonuclease activity further released the terminal 2',3'-cyclic guanosine monophosphate (2',3'-cGMP) to engage pocket 1 and was also needed to generate RNA fragments for pocket 2. Loss-of-function studies in cell lines and primary cells confirmed the critical requirement for PLD activity. Biochemical and structural studies showed that PLD enzymes form homodimers with two ligand-binding sites important for activity. Previously identified disease-associated PLD mutants failed to form stable dimers. Together, our data provide a mechanistic basis for the detection of RNA fragments by TLR7.

RNA-Templated Peptide Bond Formation Promotes L-Homochirality.

The world in which we live is homochiral. The ribose units that form the backbone of DNA and RNA are all D-configured and the encoded amino acids that comprise the proteins of all living species feature an all-L-configuration at the α-carbon atoms. The homochirality of α-amino acids is essential for folding of the peptides into well-defined and functional 3D structures and the homochirality of D-ribose is crucial for helix formation and base-pairing. The question of why nature uses only encoded L-α-amino acids is not understood. Herein, we show that an RNA-peptide world, in which peptides grow on RNAs constructed from D-ribose, leads to the self-selection of homo-L-peptides, which provides a possible explanation for the homo-D-ribose and homo-L-amino acid combination seen in nature.

Efficient Tandem Copper-Catalyzed Click Synthesis of Multisugar-Modified Oligonucleotides.

Nucleic acids in the form of siRNA, antisense oligonucleotides or mRNA are currently explored as new promising modalities in the pharmaceutical industry. Particularly, the success of mRNA-vaccines against SARS-CoV-2, along with the successful development of the first sugar-modified siRNA therapeutics has inspired the field. The development of nucleic acid therapeutics requires efficient chemistry to link oligonucleotides to chemical structures that can improve stability, boost cellular uptake, or enable specific targeting. For the siRNA therapeutics currently in use, modification of the 3'-end of the oligonucleotides with triple-N-acetylgalactosamine (GalNAc)3 was shown to be of significance. This modification is currently achieved through cumbersome multistep synthesis and subsequent loading onto the solid support material. Herein, we report the development of a bifunctional click-reactive linker that allows the modification of oligonucleotides in a tandem click reaction with multiple sugars, regardless of the position within the oligonucleotide, with remarkable efficiency and in a one-pot reaction.

Gradual evolution of a homo-l-peptide world on homo-d-configured RNA and DNA.

Modern life requires the translation of genetic information - encoded by nucleic acids - into proteins, which establishes the essential link between genotype and phenotype. During translation, exclusively l-amino acids are loaded onto transfer RNA molecules (tRNA), which are then connected at the ribosome to give homo-l-proteins. In contrast to the homo-l-configuration of amino acids and proteins, the oligonucleotides involved are all d-configured (deoxy)ribosides. Previously, others and us have shown that if peptide synthesis occurs at homo d-configured oligonucleotides, a pronounced l-amino acid selectivity is observed, which reflects the d-sugar/l-amino acid world that evolved in nature. Here we further explore this astonishing selectivity. We show a peptide-synthesis/recapture-cycle that can lead to a gradual enrichment and hence selection of a homo-l-peptide world. We show that even if peptides with a mixed l/d-stereochemistry are formed, they are not competitive against the homo-l-counterparts. We also demonstrate that this selectivity is not limited to RNA but that peptide synthesis on DNA features the same l-amino acid preference. In total, the data bring us a step closer to an understanding of how homochirality on Earth once evolved.

Generation of densely labeled oligonucleotides for the detection of small genomic elements.

The genome contains numerous regulatory elements that may undergo complex interactions and contribute to the establishment, maintenance, and change of cellular identity. Three-dimensional genome organization can be explored with fluorescence in situ hybridization (FISH) at the single-cell level, but the detection of small genomic loci remains challenging. Here, we provide a rapid and simple protocol for the generation of bright FISH probes suited for the detection of small genomic elements. We systematically optimized probe design and synthesis, screened polymerases for their ability to incorporate dye-labeled nucleotides, and streamlined purification conditions to yield nanoscopy-compatible oligonucleotides with dyes in variable arrays (NOVA probes). With these probes, we detect genomic loci ranging from genome-wide repetitive regions down to non-repetitive loci below the kilobase scale. In conclusion, we introduce a simple workflow to generate densely labeled oligonucleotide pools that facilitate detection and nanoscopic measurements of small genomic elements in single cells.