SUMMARY The regulation of protein-coding and noncoding RNAs is linked to nuclear processes including chromatin modifications and gene silencing. However, the mechanisms that distinguish RNAs and mediate their functions are poorly understood. We describe a nuclear RNA processing network in fission yeast with a core module comprising the Mtr4-like protein, Mtl1, and the zinc finger protein, Red1. The Mtl1-Red1 core promotes degradation of mRNAs and noncoding RNAs, and associates with different proteins to assemble heterochromatin via distinct mechanisms. Mtl1 also forms Red1-independent interactions with evolutionarily conserved proteins named Nrl1 and Ctr1, which associate with splicing factors. Whereas Nrl1 targets transcripts with cryptic introns to form heterochromatin at developmental genes and retrotransposons, Ctr1 functions in processing intron-containing telomerase RNA. Together with our discovery of widespread cryptic introns, including in noncoding RNAs, these findings reveal unique cellular strategies for recognizing regulatory RNAs and coordinating their functions in response to developmental and environmental cues.

SUMMARY Erh1, the fission yeast homolog of Enhancer of rudimentary, is implicated in meiotic mRNA elimination during vegetative growth, but its function is poorly understood. We show that Erh1 and the RNA-binding protein Mmi1 form a stoichiometric complex called EMC (Erh1-Mmi1 complex), to promote meiotic mRNA decay and facultative heterochromatin assembly. To perform these functions, EMC associates with two distinct complexes, MTREC (Mtl1-Red1 core) and CCR4-NOT. Whereas MTREC facilitates assembly of heterochromatin islands coating meiotic genes silenced by the nuclear exosome, CCR4-NOT promotes RNAi-dependent heterochromatin domain (HOOD) formation at EMC-target loci. CCR4-NOT also assembles HOODs at retrotransposons and regulated genes containing cryptic introns. We find that CCR4-NOT facilitates HOOD assembly through its association with the conserved Pir2/ARS2 protein, and also maintains rDNA integrity and silencing by promoting heterochromatin formation. Our results reveal connections among Erh1, CCR4-NOT, Pir2/ARS2 and RNAi, which target heterochromatin to regulate gene expression and to protect genome integrity.

Cotranscriptional RNA processing and surveillance factors mediate heterochromatin formation in diverse eukaryotes. In fission yeast, RNAi machinery and RNA elimination factors including the Mtl1-Red1 core and the exosome are involved in facultative heterochromatin assembly; however, the exact mechanisms remain unclear. Here we show that RNA elimination factors cooperate with the conserved exoribonuclease Dhp1/Rat1/Xrn2, which couples premRNA 3′-end processing to transcription termination, to promote premature termination and facultative heterochromatin formation at meiotic genes. We also find that Dhp1 is critical for RNAi-mediated heterochromatin assembly at retroelements and regulated gene loci and facilitates the formation of constitutive heterochromatin at centromeric and mating-type loci. Remarkably, our results reveal that Dhp1 interacts with the Clr4/Suv39h methyltransferase complex and acts directly to nucleate heterochromatin. Our work uncovers a previously unidentified role for 3′-end processing and transcription termination machinery in gene silencing through premature termination and suggests that noncanonical transcription termination by Dhp1 and RNA elimination factors is linked to heterochromatin assembly. These findings have important implications for understanding silencing mechanisms targeting genes and repeat elements in higher eukaryotes.heterochromatin | Dhp1/Xrn2 | S. pombe | transcription termination

Group II introns are self-splicing RNAs found in eubacteria, archaea, and eukaryotic organelles. They are mechanistically similar to the metazoan nuclear spliceosomal introns; therefore, group II introns have been invoked as the progenitors of the eukaryotic pre-mRNA introns. However, the ability of group II introns to function outside of the bacteria-derived organelles is debatable, since they are not found in the nuclear genomes of eukaryotes. Here, we show that the Lactococcus lactis Ll.LtrB group II intron splices accurately and efficiently from different pre-mRNAs in a eukaryote, Saccharomyces cerevisiae. However, a pre-mRNA harboring a group II intron is spliced predominantly in the cytoplasm and is subject to nonsense-mediated mRNA decay (NMD), and the mature mRNA from which the group II intron is spliced is poorly translated. In contrast, a pre-mRNA bearing the Tetrahymena group I intron or the yeast spliceosomal ACT1 intron at the same location is not subject to NMD, and the mature mRNA is translated efficiently. Thus, a group II intron can splice from a nuclear transcript, but RNA instability and translation defects would have favored intron loss or evolution into protein-dependent spliceosomal introns, consistent with the bacterial group II intron ancestry hypothesis. Group II introns are self-splicing, mobile ribozymes that naturally inhabit diverse genes in the genomes of bacteria and eukaryotic organelles (Belfort et al. 2002;Dai et al. 2003;Pyle and Lambowitz 2006). They often encode a protein that, inter alia, helps fold the group II intron into its characteristic secondary and tertiary structure, required for its catalysis. These autocatalytic group II introns are mechanistically similar to the eukaryotic nuclear pre-mRNA introns, which require a dynamic spliceosomal complex consisting of five indispensable small nuclear RNAs (snRNAs) to catalyze splicing (Grabowski et al. 1985;Moore and Sharp 1993;Padgett et al. 1994;Sontheimer et al. 1999;Patel and Steitz 2003). Additionally, these snRNAs have structural and functional similarities to certain domains of group II introns, and boundary sequences between the group II and spliceosomal introns are similar (Cech 1986;Hetzer et al. 1997;Shukla and Padgett 2002;Toor et al. 2008;Keating et al. 2010).Because of the parallels between bacterial group II and eukaryotic spliceosomal introns, the catalytic group II introns have been proposed to be the progenitors of the eukaryotic spliceosomal introns (Cech 1986; CavalierSmith 1991;Sharp 1991;Lynch and Kewalramani 2003;Martin and Koonin 2006;Roy and Gilbert 2006). It is widely speculated that group II introns entered the eukaryotic lineage with the mitochondrial endosymbiosis, invaded the nucleus, and evolved into more efficient spliceosome-dependent introns. However, unlike spliceosomal introns, group II introns are not found in the proteincoding genes of nuclear genomes. Also, there is no evidence for the functioning of group II introns outside of the bacteria-derived mitochondria and chloroplasts. It is t...