Exon-activated functional rescue secures neuronal transcript signatures

The production of alternative RNA variants contributes to the tissue-specific regulation of gene expression. In the animal nervous system, a shift towards more distal 3’ processing sites generates hundreds of substantially extended messenger RNAs. These neuronal transcript signatures are crucial for nervous system development and function. However, no clear understanding of the mechanism of systematic 3’ extension in neurons has yet been achieved. Here, we report that the highly conserved RNA-binding protein ELAV globally regulates all events of neuronal 3’ end processing by directly binding to the proximal 3’ end of its target RNAs. We uncover an endogenous strategy of functional gene rescue, by which neuronal RNA signatures are safeguarded in a loss-of-function context. When not directly repressed by ELAV, the transcript encoding the ELAV paralogue FNE acquires a mini‑exon, which equips the new FNE protein with the ability to translocate to the nucleus and rescue neuronal 3’ end processing. We propose that exon-activated functional rescue is a widespread mechanism that ensures robustness of processes that are regulated by a hierarchy, rather than by redundancy, of effectors. Analysis of alternative polyadenyation using RNA-seq and Clip-seq data