Skip to main content
Chapter 5

Silent Editing

Bacterial RADAR, human ADAR1, and the pharmacology of RNA masking

“In humans, ADAR1 disguises endogenous RNA so it doesn't start a war. In bacteria, RADAR edits viral RNA so it no longer functions at all. Same enzyme, two strategies.”

Narrative

A chemistry detail that changes everything

In a messenger RNA sequence, some adenines (A) are silently replaced by inosines (I). This modification—A→I editing—is invisible if sequenced without specific precautions (inosine is read as a guanine by most polymerases). It has massive consequences:

  • The edited RNA changes behavior when entering the ribosome (inosine is read as G, modifying the codon).
  • The edited RNA is no longer recognized as foreign by double-stranded RNA sensors (MDA5, RIG-I).
  • The edited RNA no longer has the same secondary structure (base pairing formation changes).

In mammals, the responsible enzyme is ADAR1. It has two missions that may seem contradictory:

  1. Mask endogenous double-stranded RNA so MDA5 does not confuse it with viral RNA and trigger an interferonopathy.
  2. Edit viral RNA when caught. This can either inactivate it (non-functional transcript) or—paradoxically—be hijacked by some viruses to escape detection.

It is a double-edged enzyme. Too much ADAR1 → RNA viruses escape by hiding behind the editing. Not enough ADAR1 → chronic interferonopathy like Aicardi-Goutières. The therapeutic window to modulate it is narrow. The stakes are immense.

The RADAR discovery

The RADAR system (Restriction by an Adenosine Deaminase Acting on RNA) was characterized in 2023 by Duncan-Lowey et al. in Cell (DOI 10.1016/j.cell.2023.10.013). The cryo-EM structure is stunning. It is a giant supramolecular assembly composed of an ATPase (RdrA) and a deaminase (RdrB) that self-organize into filaments.

The homology with ADAR1 is striking: same catalytic deaminase fold, same A→I deamination mechanism, same substrate (double-stranded RNA). It is one of the most beautiful functional homologies in the entire anti-phage arsenal characterized by 2026.

The difference: bacteria use this enzyme massively against phages. The edited viral RNAs become untranslatable, and the phage aborts. It is an aggressive use of A→I editing, where humans use it defensively/for masking.

The Bactaegion opportunity

Here is the bet. If we can identify allosteric pockets conserved with ADAR1 on the non-catalytic surface of bacterial RdrB, we can bring out transferable pharmacological probes.

The goal is not an antibacterial. It is to modulate human ADAR1 finely to:

  • Expose viral RNAs in cells infected by HIV, HCV, flaviviruses—awakening the interferon response without triggering chronic interferonopathy.
  • Sensitize tumors that overuse ADAR1 to mask endogenous double-stranded RNA and escape cell death. This is a central mechanism in several cancers resistant to current therapies.

This is explicitly a host-directed strategy. The target is our own enzyme, not a viral protein. The bacterium merely serves as a structural template. It is simpler, more exploitable, without the cryptic allostery problem of human ADAR1.

The RdrB → ADAR1 Modulators lead proposes a concrete plan: structural alignment via Mol* + DALI, FPocket identification on non-catalytic surfaces, ZINC20 lead-like screening, and selectivity filtering vs cerebral ADAR2.

Why this bet is delicate

Three traps to avoid:

  1. ADAR1 allostery is poorly mapped. The few published inhibitors are competitive at the catalytic site (thus non-selective between ADAR1 and ADAR2). Finding an exploitable allosteric pocket is precisely the stake of this lead—and its main risk.

  2. The RdrB → ADAR1 transfer assumes conservation of regulatory sites beyond the catalytic site. It is plausible (the deaminase fold is highly conserved), but not guaranteed.

  3. ADAR1 inhibition carries a real autoimmunity risk (cf. Aicardi-Goutières). The realistic strategy is a reversible, short-acting inhibitor for acute treatment—not chronic modulation.

This is exactly the kind of hypothesis to falsify collectively. If your intuition tells you the RdrB and ADAR1 pockets do not resemble each other enough, open the verdict of this lead on the dedicated page and express your reservation.

To go further

All chapters Annotate a protein now Read the leads