Since the discovery of splicing 40 years ago, no function has been reported for intact excised introns; they are believed to be degraded within seconds of being spliced-out. In this article, David P. Bartel and co-authors report a surprising discovery that 34 of the 300 S.cerevisiae introns are stabilized as intact, linear RNAs when cells transition from log-phase to saturated growth, or when cells are exposed to other stresses that result in prolonged inhibition of TOR, a key integrator of growth signalling. The stabilization of introns is independent of their genomic origin or sequence and requires a short distance between their lariat branch point and 3′ splice site. Stable introns remain associated with components of the post-catalytic spliceosome and are biochemically coupled to TORC1 inhibition, via an unknown molecular pathway. Thus, this study shows that spliced-out introns—previously considered to be inactive byproducts of exon ligation—have an important biological function in controlling the yeast TOR growth-signalling network.
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