Flipons, Noncoding Transcripts and Retroelements: A Universal Host Defense

Encoding genetic information involves both structure and sequence. Non-protein-coding (NCR) RNAs modulate host immune responses by guiding and assembling stress-specific protein effectors, with flipons enabling dynamic transitions between states. Small NCRs transcribed by both RNA Polymerases 2 and 3 normally modulate these pathways. The miRNAs, tRNA fragments, mitrons, piRNAs, and endo-small-interfering RNA products target the guide effector complexes to their targets. These NCRs filter the subset of RNAs that are translated, a process modulated by other NCRs that fine-tune RNA stability, localization, and kinetics through the modifications they direct. When these RNAs, or their targets, are dysregulated, host interferon responses are activated through the accumulation of uncapped triphosphorylated RNAs, dsRNAs, or flipon-encoded Z-RNAs. These responses are initially suppressed to avoid attacks on self-RNAs, but above a threshold, highly immunogenic pathways are unleashed. These same responses resist viral and other infections without pathogen-specific effectors or prior exposure. They also eliminate dysfunctional cells. These universal responses, when unresolved, contribute to a range of pathologies, increasing cancer, inflammatory diseases, and senescence. The review focuses on recent findings showing how these integrated host responses are driven by, and exploit, the retrotransposon invasion of animal genomes.