Trans-splicing of the C. elegans let-7 primary transcript developmentally regulates let-7 microRNA biogenesis and let-7 family microRNA activity

let-7 is a microRNA whose sequence and roles as a regulator of developmental progression are conserved throughout bilaterians. In most systems, transcription of the let-7 locus occurs relatively early in development, whilst processing of let-7 primary transcript into mature microRNA arises later and is associated with cellular differentiation. In C. elegans and other animals, the RNA binding protein LIN-28 post-transcriptionally inhibits let-7 biogenesis at early developmental stages. The mechanisms by which LIN-28 and other factors developmentally regulate let-7 biogenesis are not fully understood. Nor is it understood how the developmental regulation of let-7 might influence the expression or activities of other microRNAs of the same seed family. Here we show that in C. elegans, the primary let-7 transcript (pri-let-7) is trans-spliced to SL1 splice leader at a position downstream of the let-7 precursor stem-loop, producing a short, polyadenylated downstream mRNA. The trans-splicing event negatively impacts the biogenesis of mature let-7 microRNA in cis, likely by destabilizing the upstream pri-let-7 fragment. Moreover, the trans-spliced downstream mRNA contains complimentary sequences to multiple members of the let-7 seed family (let-7fam), and thereby serves as a sponge to negatively regulate let-7fam function in trans. Thus, this study provides evidence for a mechanism by which splicing of a microRNA primary transcript can negatively regulate said microRNA in cis as well as other microRNAs in trans.\n\nHIGHLIGHTSO_LIThe let-7 primary transcript is trans-spliced to produce an RNA that functions as a sponge that negatively regulates the let-7-family microRNAs.\nC_LIO_LITrans-splicing of this RNA negatively impacts let-7 microRNA biogenesis.\nC_LIO_LILIN-28 regulates this trans-splicing event\nC_LI