Chemical-Guided SHAPE Sequencing Illuminates RNA Degrader Binding in SARS-CoV-2 5’ UTR

Study Background and Research Question

RNA viruses, including SARS-CoV-2, rely on complex RNA structures in their untranslated regions (UTRs) for key stages of their lifecycle. The 5’ UTR, in particular, contains multiple conserved stem-loops (SL1–SL5) that orchestrate viral translation, transcription, and packaging. Given the 5’ UTR’s functional importance and structural conservation, it presents an attractive target for antiviral intervention. However, designing small molecules or chimeric degraders that selectively bind and disrupt these RNA elements requires precise mapping of ligand binding sites—a challenge compounded by RNA flexibility and the resolution limits of traditional footprinting methods. The referenced study (Tang et al., 2025) set out to overcome this bottleneck by developing a sequencing-based approach to directly locate small-molecule binding sites within structured viral RNAs.

Key Innovation from the Reference Study

The central innovation of the work by Tang et al. is the development of chemical-guided SHAPE sequencing (cgSHAPE-seq), a method that enables high-resolution identification of small molecule binding sites on RNA. Unlike conventional structural probing, which often infers binding sites indirectly, cgSHAPE-seq leverages a specially designed acylating probe conjugated to the RNA ligand. Upon binding, the probe selectively acylates the 2’-OH group of ribose at the interaction site, marking the precise nucleotide contacted by the ligand. These acylation events are subsequently read out as mutation signatures during reverse transcription and identified using next-generation sequencing. This approach not only pinpoints the binding site but also quantifies site occupancy, streamlining structure-activity relationship (SAR) analyses for RNA-targeted therapeutics.

Methods and Experimental Design Insights

The cgSHAPE-seq workflow comprises four main steps ( see Fig. 1b):

• Probe Synthesis: Coumarin derivatives (identified as RNA-binding warheads) are chemically conjugated to a selective acylation moiety, forming a bifunctional probe.
• Targeted Acylation: The probe is incubated with in vitro transcribed SARS-CoV-2 5’ UTR RNA, allowing specific acylation at ligand-bound 2’-OH positions.
• Reverse Transcription and Mutational Profiling: Reverse transcription is performed in the presence of Mn²⁺, which promotes read-through at acylated nucleotides, resulting in single-nucleotide mutations at modification sites.
• Sequencing and Data Analysis: Mutations are mapped by high-throughput sequencing, revealing the precise locations of ligand-RNA contacts.

This method was applied to a library of coumarin derivatives screened for binding to the SL5 region of the SARS-CoV-2 5’ UTR. Mutational analysis highlighted a bulged guanosine within the four-way junction of SL5 as the major binding site, a finding validated by site-directed mutagenesis and in vitro binding assays.

Protocol Parameters

• RNA Preparation: In vitro transcription of the SARS-CoV-2 5’ UTR (or subdomains such as SL5) under RNase-free conditions. RNA is typically purified by denaturing PAGE and stored in RNase-free water.
• Probe Incubation: Incubate 1–5 μM RNA with 10–20-fold molar excess of coumarin-acylating probe at 25–37°C for 30–60 min in structure-stabilizing buffer (e.g., 50 mM HEPES, 100 mM KCl, 5 mM MgCl₂).
• Reverse Transcription: Use Mn²⁺ (typically 0.5–2 mM) to facilitate read-through at acylated nucleotides; primer extension reactions can be performed with thermostable reverse transcriptase for improved fidelity.
• Sequencing Library Prep: PCR amplify cDNA for next-generation sequencing, ensuring coverage of the full RNA region of interest.
• RNase Inhibition (Practical Suggestion): Include a murine RNase inhibitor during all RNA handling steps to prevent adventitious RNA degradation, especially during probe incubation and reverse transcription (internal review).

Core Findings and Why They Matter

The application of cgSHAPE-seq revealed that coumarin derivatives selectively bind a bulged guanosine within the highly conserved SL5 element of the SARS-CoV-2 5’ UTR. This region is not only structurally stable across diverse viral lineages but also functionally pivotal for viral translation and transcription. Armed with this binding-site knowledge, the authors engineered RNA-degrading chimeras (RIBOTACs) that recruit endogenous RNase L to the viral RNA, promoting its degradation. Notably, the optimized chimera (C64) effectively reduced SARS-CoV-2 RNA levels and inhibited replication in infected human lung epithelial cells. This result underscores the therapeutic potential of rationally designed RNA-targeted degraders guided by high-resolution binding maps (Tang et al., 2025).

On a broader scale, cgSHAPE-seq provides a generalizable platform for mapping ligand interactions on structured RNAs, which could accelerate the development of RNA-targeted therapeutics beyond virology, including in cancer and rare genetic diseases where RNA structure is functionally relevant.

Comparison with Existing Internal Articles

Several internal resources contextualize the importance of robust RNA degradation prevention in advanced molecular workflows. For example, the article “Murine RNase Inhibitor: Redefining RNA Degradation Prevention” discusses how maintaining RNA integrity is crucial for techniques like in vitro transcription and real-time RT-PCR, both of which are foundational for cgSHAPE-seq and similar structural mapping methods. Further, “Murine RNase Inhibitor: Oxidation-Resistant RNA Protection” highlights the specific advantages of murine RNase inhibitors in workflows that require high oxidative stability, such as those involving minimal DTT concentrations or long incubation steps. These articles emphasize that the reliability of advanced RNA mapping and degradation assays depends on rigorous RNA degradation prevention, especially when handling structured viral RNAs prone to nuclease attack.

Limitations and Transferability

While cgSHAPE-seq offers unprecedented resolution for mapping ligand binding sites on structured RNA, its application is currently limited by the need for the chemical synthesis of specialized acylating probes and the requirement for in vitro RNA preparation. The method’s reliance on next-generation sequencing infrastructure may also present accessibility barriers for some laboratories. Transferability to highly modified or cellular RNAs will require further validation, as endogenous RNA modifications or protein binding could obscure acylation patterns. Nevertheless, the approach is well poised for adaptation to other viral or cellular RNAs where high-resolution binding-site data is needed for therapeutic targeting.

Research Support Resources

To ensure the highest RNA integrity during cgSHAPE-seq or related workflows, researchers can incorporate Murine RNase Inhibitor (SKU K1046) as a selective RNase A inhibitor. Its oxidation-resistant properties and efficacy at low DTT concentrations make it a reliable choice for RNA degradation prevention in demanding molecular biology protocols, including real-time RT-PCR, cDNA synthesis, and in vitro transcription. For further background on best practices for RNA protection and workflow optimization, refer to the in-depth article “Murine RNase Inhibitor: Precision RNA Protection in RT-PCR”.