The sequence and roles in developmental progression of the microRNA
let-7 are conserved. In many animals, including C. elegans, transcription of the
let-7 primary transcript (pri-
let-7) occurs throughout development, whereas processing of the mature
let-7 microRNA arises during cellular differentiation. LIN-28 is a conserved RNA binding protein that negative regulates
let-7 biogenesis. In mammals, Lin28 achieves this negative regulation by binding the
let-7/let-7* stem loop of pri-
let-7 or pre-
let-7 to physically prevent microprocessing. In C. elegans, it is thought that LIN-28 represses
let-7 biogenesis by binding pri-
let-7 downstream of the
let-7/let-7* stem loop, but the mechanisms by which LIN-28 exerts repression upon
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. We find that LIN-28 negatively regulates
let-7 through at least two mechanisms: 1) through the predicted binding site downstream of the
let-7/let-7* stem loop, and 2) by promoting SL1trans-splicing of pri-
let-7 downstream of the
let-7 precursor stem-loop to produce a short polyadenylated mRNA. This trans-spliced mRNA contains sequences that are complementary to
let-7 seed family (
let-7fam) microRNAs, and hence can negatively regulate
let-7fam function in trans. Interestingly, compound mutations in both the predicted LIN-28 binding site and trans-splice acceptor site fail to recapitulate the increase in
let-7 observed in
lin-28 null animals suggesting additional unknown mechanism(s) of LIN-28-mediated negative regulation on
let-7 processing. Thus, this study highlights the complexity of
let-7 biogenesis by LIN-28 and describes a novel mechanism by which LIN-28 negatively regulates multiple related microRNAs.