Introns encode a vast new class of Kink-loop RNAs that autoregulate pre-mRNA splicing

Introns occupy nearly one-third of the human genome, yet whether they encode widespread regulatory functions remains unclear. Here, we identify a vast new class of intron-derived Kink-loop RNAs (klRNAs) that autoregulate host pre-mRNA splicing. We developed orthogonal sequencing methods to systematically uncover approximately 15,200 and 3,500 previously unannotated klRNAs in humans and mice, respectively. Bound by the conserved RNA-binding protein 15.5K, klRNAs are compact orphan RNAs characterized by stereotypically positioned terminal C/D motifs that form K-loop structures. Depletion of 15.5K broadly disrupts klRNA biogenesis. Functional and genetic perturbations establish that klRNAs suppress host intron excision through a K-loop-dependent mechanism. Together, our findings establish klRNA-mediated autoregulation as a widespread principle governing intron fate and reveal a previously unrecognized regulatory layer encoded within mammalian introns.