The Drosha–DGCR8, or Microprocessor, complex, is required for microRNA biogenesis. DGCR8 recognizes the RNA substrates, whereas Drosha functions as an endonuclease. High-throughput sequencing and cross-linking immunoprecipitation (HITS-CLIP) analyses reveal many other RNA targets of DGCR8 besides microRNAs, including snoRNAs, which are processed in a Drosha-independent manner, as well as long noncoding RNAs and mRNAs of protein-coding genes. The Drosha–DGCR8 complex (Microprocessor) is required for microRNA (miRNA) biogenesis. DGCR8 recognizes the RNA substrate, whereas Drosha functions as the endonuclease. Using high-throughput sequencing and cross-linking immunoprecipitation (HITS-CLIP) we identified RNA targets of DGCR8 in human cells. Unexpectedly, miRNAs were not the most abundant targets. DGCR8-bound RNAs also comprised several hundred mRNAs as well as small nucleolar RNAs (snoRNAs) and long noncoding RNAs. We found that the Microprocessor controlled the abundance of several mRNAs as well as of MALAT1. By contrast, DGCR8-mediated cleavage of snoRNAs was independent of Drosha, suggesting the involvement of DGCR8 in cellular complexes with other endonucleases. Binding of DGCR8 to cassette exons is a new mechanism for regulation of the relative abundance of alternatively spliced isoforms. These data provide insights in the complex role of DGCR8 in controlling the fate of several classes of RNAs.

How the activity of transposable elements is regulated is not well understood. A new study now shows that the Microprocessor complex, which is required for microRNA biogenesis, also recognizes and binds RNAs derived from human LINE-1, Alu and SVA retrotransposons and that it acts as a post-transcriptional repressor of mammalian retrotransposons in vivo. More than half of the human genome is made of transposable elements whose ongoing mobilization is a driving force in genetic diversity; however, little is known about how the host regulates their activity. Here, we show that the Microprocessor (Drosha-DGCR8), which is required for microRNA biogenesis, also recognizes and binds RNAs derived from human long interspersed element 1 (LINE-1), Alu and SVA retrotransposons. Expression analyses demonstrate that cells lacking a functional Microprocessor accumulate LINE-1 mRNA and encoded proteins. Furthermore, we show that structured regions of the LINE-1 mRNA can be cleaved in vitro by Drosha. Additionally, we used a cell culture–based assay to show that the Microprocessor negatively regulates LINE-1 and Alu retrotransposition in vivo. Altogether, these data reveal a new role for the Microprocessor as a post-transcriptional repressor of mammalian retrotransposons and a defender of human genome integrity.

Around half of the genomes in mammals are composed of transposable elements (TEs) such as DNA transposons and retrotransposons. Several mechanisms have evolved to prevent their activity and the detrimental impact of their insertional mutagenesis. Despite these potentially negative effects, TEs are essential drivers of evolution, and in certain settings, beneficial to their hosts. For instance, TEs have rewired the antiviral gene regulatory network and are required for early embryonic development. However, due to structural similarities between TE-derived and viral nucleic acids, cells can misidentify TEs as invading viruses and trigger the major antiviral innate immune pathway, the type I interferon (IFN) response. This review will focus on the different settings in which the role of TE-mediated IFN activation has been documented, including cancer and senescence. Importantly, TEs may also play a causative role in the development of complex autoimmune diseases characterized by constitutive type I IFN activation. All these observations suggest the presence of strong but opposing forces driving the coevolution of TEs and antiviral defense. A better biological understanding of the TE replicative cycle as well as of the antiviral nucleic acid sensing mechanisms will provide insights into how these two biological processes interact and will help to design better strategies to treat human diseases characterized by aberrant TE expression and/or type I IFN activation.

Cell-based studies of human ribonucleases traditionally rely on methods that deplete proteins slowly. We engineered cells in which the 3'→5' exoribonucleases of the exosome complex, DIS3 and EXOSC10, can be rapidly eliminated to assess their immediate roles in nuclear RNA biology. The loss of DIS3 has the greatest impact, causing the substantial accumulation of thousands of transcripts within 60 min. These transcripts include enhancer RNAs, promoter upstream transcripts (PROMPTs), and products of premature cleavage and polyadenylation (PCPA). These transcripts are unaffected by the rapid loss of EXOSC10, suggesting that they are rarely targeted to it. More direct detection of EXOSC10-bound transcripts revealed its substrates to prominently include short 3' extended ribosomal and small nucleolar RNAs. Finally, the 5'→3' exoribonuclease, XRN2, has little activity on exosome substrates, but its elimination uncovers different mechanisms for the early termination of transcription from protein-coding gene promoters.

The Microprocessor complex (DGCR8/Drosha) is required for microRNA (miRNA) biogenesis but also binds and regulates the stability of several types of cellular RNAs. Of particular interest, DGCR8 controls the stability of mature small nucleolar RNA (snoRNA) transcripts independently of Drosha, suggesting the existence of alternative DGCR8 complex(es) with other nucleases to process a variety of cellular RNAs. Here, we found that DGCR8 copurifies with subunits of the nuclear exosome, preferentially associating with its hRRP6-containing nucleolar form. Importantly, we demonstrate that DGCR8 is essential for the recruitment of the exosome to snoRNAs and to human telomerase RNA. In addition, we show that the DGCR8/exosome complex controls the stability of the human telomerase RNA component (hTR/TERC). Altogether, these data suggest that DGCR8 acts as an adaptor to recruit the exosome complex to structured RNAs and induce their degradation.

MiRNA biogenesis is highly regulated at the post-transcriptional level; however, the role of sequence and secondary RNA structure in this process has not been extensively studied. A single G to A substitution present in the terminal loop of pri-mir-30c-1 in breast and gastric cancer patients had been previously described to result in increased levels of mature miRNA. Here, we report that this genetic variant directly affects Drosha-mediated processing of pri-mir-30c-1 in vitro and in cultured cells. Structural analysis of this variant revealed an altered RNA structure that facilitates the interaction with SRSF3, an SR protein family member that promotes pri-miRNA processing. Our results are compatible with a model whereby a genetic variant in pri-mir-30c-1 leads to a secondary RNA structure rearrangement that facilitates binding of SRSF3 resulting in increased levels of miR-30c. These data highlight that primary sequence determinants and RNA structure are key regulators of miRNA biogenesis.