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The MODOMICS database has been, since 2006, a manually curated and centralized resource, storing and distributing comprehensive information about modified ribonucleosides. Originally, it only contained data on the chemical structures of modified ribonucleosides, their biosynthetic pathways, the location of modified residues in RNA sequences, and RNA-modifying enzymes. Over the years, prompted by the accumulation of new knowledge and new types of data, it has been updated with new information and functionalities. In this new release, we have created a catalog of RNA modifications linked to human diseases, e.g., due to mutations in genes encoding modification enzymes. MODOMICS has been linked extensively to RCSB Protein Data Bank, and sequences of experimentally determined RNA structures with modified residues have been added. This expansion was accompanied by including nucleotide 5′-monophosphate residues. We redesigned the web interface and upgraded the database backend. In addition, a search engine for chemically similar modified residues has been included that can be queried by SMILES codes or by drawing chemical molecules. Finally, previously available datasets of modified residues, biosynthetic pathways, and RNA-modifying enzymes have been updated. Overall, we provide users with a new, enhanced, and restyled tool for research on RNA modification. MODOMICS is available at https://iimcb.genesilico.pl/modomics/.

Base editing is an exciting new genome engineering technology. C-to-T mutations in genomic DNA have been achieved using ribonucleoprotein complexes comprised of rat APOBEC1 single-stranded DNA deaminase, Cas9 nickase (Cas9n), uracil DNA glycosylase inhibitor (UGI), and guide (g)RNA. Here, we report the first real-time reporter system for quantification of APOBEC-mediated base editing activity in living mammalian cells. The reporter expresses eGFP constitutively as a marker for transfection or transduction, and editing restores functionality of an upstream mCherry cassette through the simultaneous processing of two gRNA binding regions that each contain an APOBEC-preferred 5′TCA target site. Using this system as both an episomal and a chromosomal editing reporter, we show that human APOBEC3A-Cas9n-UGI and APOBEC3B-Cas9n-UGI base editing complexes are more efficient than the original rat APOBEC1-Cas9n-UGI construct. We also demonstrate coincident enrichment of editing events at a heterologous chromosomal locus in reporter-edited, mCherry-positive cells. The mCherry reporter also quantifies the double-stranded DNA cleavage activity of Cas9, and may therefore be adaptable for use with many different CRISPR systems. The combination of a rapid, fluorescence-based editing reporter system and more efficient, structurally defined DNA editing enzymes broadens the versatility of the rapidly expanding toolbox of genome editing and engineering technologies.

Human APOBEC3F and APOBEC3G are double-domained deaminases that can catalyze dC--dU deamination in HIV-1 and MLV retroviral DNA replication intermediates, targeting T-C or C-C dinucleotides, respectively. HIV-1 antagonizes their action through its vif gene product, which has been shown (at least in the case of APOBEC3G) to interact with the N-terminal domain of the deaminase, triggering its degradation. Here, we compare APOBEC3F and APOBEC3G to APOBEC3C, a single-domained deaminase that can also act on both HIV-1 and MLV. We find that whereas APOBEC3C contains all the information necessary for both Vif-binding and cytidine deaminase activity in a single domain, it is the C-terminal domain of APOBEC3F and APOBEC3G that confer their target site specificity for cytidine deamination. We have exploited the fact that APOBEC3C, whilst highly homologous to the C-terminal domain of APOBEC3F, exhibits a distinct target site specificity (preferring Y-C dinucleotides) in order to identify residues in APOBEC3F that might affect its target site specificity. We find that this specificity can be altered by single amino acid substitutions at several distinct positions, suggesting that the strong dependence of APOBEC3-mediated deoxycytidine deamination on the 5'-flanking nucleotide is sensitive to relatively subtle changes in the APOBEC3 structure. The approach has allowed the isolation of APOBEC3 DNA mutators that exhibit novel target site preferences.