Explore the vast range of titles available.

Heterostyly represents a fascinating adaptation to promote outbreeding in plants that evolved multiple times independently. While L-morph individuals form flowers with long styles, short anthers, and small pollen grains, S-morph individuals have flowers with short styles, long anthers, and large pollen grains. The difference between the morphs is controlled by an S-locus “supergene” consisting of several distinct genes that determine different traits of the syndrome and are held together, because recombination between them is suppressed. In Primula, the S locus is a roughly 300-kb hemizygous region containing five predicted genes. However, with one exception, their roles remain unclear, as does the evolutionary buildup of the S locus. Here we demonstrate that the MADS-box GLOBOSA2 (GLO2) gene at the S locus determines anther position. In Primula forbesii S-morph plants, GLO2 promotes growth by cell expansion in the fused tube of petals and stamen filaments beneath the anther insertion point; by contrast, neither pollen size nor male incompatibility is affected by GLO2 activity. The paralogue GLO1, from which GLO2 arose by duplication, has maintained the ancestral B-class function in specifying petal and stamen identity, indicating that GLO2 underwent neofunctionalization, likely at the level of the encoded protein. Genetic mapping and phylogenetic analysis indicate that the duplications giving rise to the style-length-determining gene CYP734A50 and to GLO2 occurred sequentially, with the CYP734A50 duplication likely the first. Together these results provide the most detailed insight into the assembly of a plant supergene yet and have important implications for the evolution of heterostyly.

The exosome is the major 3′–5′ RNA-degradation complex in eukaryotes. The ubiquitous core of the yeast exosome (Exo-10) is formed by nine catalytically inert subunits (Exo-9) and a single active RNase, Rrp44. In the nucleus, the Exo-10 core recruits another nuclease, Rrp6. Here we crystallized an approximately 440-kilodalton complex of Saccharomyces cerevisiae Exo-10 bound to a carboxy-terminal region of Rrp6 and to an RNA duplex with a 3′-overhang of 31 ribonucleotides. The 2.8 Å resolution structure shows how RNA is funnelled into the Exo-9 channel in a single-stranded conformation by an unwinding pore. Rrp44 adopts a closed conformation and captures the RNA 3′-end that exits from the side of Exo-9. Exo-9 subunits bind RNA with sequence-unspecific interactions reminiscent of archaeal exosomes. The substrate binding and channelling mechanisms of 3′–5′ RNA degradation complexes are conserved in all kingdoms of life. The crystal structure of a complete yeast exosome (Exo-10) bound to a region of the Rrp6 nuclease and an RNA substrate is determined, demonstrating that the exosome binds and degrades RNA molecules with a channelling mechanism that is largely conserved in all kingdoms of life and is similar to the mechanism used by the proteasome to degrade polypeptides. The exosome preparing to take a bite out of RNA The major factor responsible for RNA turnover is a ten-subunit complex known as the exosome. This complex contains a single active RNase subunit, Rrp44. In the nucleus the exosome recruits an accessory factor that is also a nuclease, Rrp6. This study reports the 2.8 Å resolution crystal structure of a 440-kilodalton complex comprising the core yeast exosome, a carboxy-terminal region of Rrp6 and an RNA duplex, visualizing how RNA is recognized and prepared for degradation.

The RNA exosome, a major RNA degradation machine, processes ribosomal RNA (rRNA) precursors and is directly coupled to the protein synthesis machine, the ribosome. Using cryo–electron microscopy, Schuller et al. investigated the structure of the precursor large ribosomal subunit from yeast with unprocessed rRNA in complex with the RNA exosome. The structure captures a snapshot of two molecular machines transiently interacting and explains how the RNA exosome acts on an authentic physiological substrate and remodels the large subunit during ribosome maturation. Science , this issue p. 219 The exosome forms a single structural and functional unit with its preribosomal substrate to catalyze ribosomal RNA processing and ribosome maturation. The RNA exosome complex processes and degrades a wide range of transcripts, including ribosomal RNAs (rRNAs). We used cryo–electron microscopy to visualize the yeast nuclear exosome holocomplex captured on a precursor large ribosomal subunit (pre-60 S ) during 7 S -to-5.8 S rRNA processing. The cofactors of the nuclear exosome are sandwiched between the ribonuclease core complex (Exo-10) and the remodeled “foot” structure of the pre-60 S particle, which harbors the 5.8 S rRNA precursor. The exosome-associated helicase Mtr4 recognizes the preribosomal substrate by docking to specific sites on the 25 S rRNA, captures the 3′ extension of the 5.8 S rRNA, and channels it toward Exo-10. The structure elucidates how the exosome forms a structural and functional unit together with its massive pre-60 S substrate to process rRNA during ribosome maturation.

The eukaryotic exosome is a conserved RNA-degrading complex that functions in RNA surveillance, turnover and processing. How the same machinery can either completely degrade or precisely trim RNA substrates has long remained unexplained. Here we report the crystal structures of a yeast nuclear exosome containing the 9-subunit core, the 3′–5′ RNases Rrp44 and Rrp6, and the obligate Rrp6-binding partner Rrp47 in complex with different RNAs. The combined structural and biochemical data of this 12-subunit complex reveal how a single-stranded RNA can reach the Rrp44 or Rrp6 active sites directly or can bind Rrp6 and be threaded via the central channel towards the distal RNase Rrp44. When a bulky RNA is stalled at the entrance of the channel, Rrp6–Rrp47 swings open. The results suggest how the same molecular machine can coordinate processive degradation and partial trimming in an RNA-dependent manner by a concerted swinging mechanism of the two RNase subunits. Solving the crystal structure of an exosome complex from yeast, bound to different RNA substrates, offers insights into how the exosome can be utilized for precise processing of some 3′ ends, such as that of the 5.8S rRNA, while other RNAs are degraded to completion. Exosome's flexible approach to RNA The main focus of RNA turnover in a cell is the exosome, a 12-subunit complex that contains two exoribonucleases, Rrp44 and Rrp6. Elena Conti and colleagues have now solved the crystal structure of this complex from yeast, bound to a variety of RNA substrates. The structures offer insight into how the exosome is used for the precise processing of some 3′ ends, such as that of the 5.8S rRNA, while other RNAs are degraded to completion. Coordination between the two nuclease subunits is seen to involve a swinging motion by Rrp6 and its partner, Rrp47.

Different strategies to reduce selfing and promote outcrossing have evolved in hermaphroditic flowers. Heterostyly, a complex floral polymorphism that occurs in at least 27 families of angiosperms, is hypothesized to achieve both goals by optimizing cross‐pollination (via disassortative pollen transfer) and restricting gamete wastage to autogamy (via the reduction in sexual interference between male and female organs). In heterostylous flowers, the reciprocal positioning of sexual organs in different morphs and the pollen incompatibility system within flower or between flowers of the same morph are thought to optimize both male and female functions, reducing the conflicts inherent to the occurrence of both sexual organs in the same reproductive unit. Specific elements of the disassortative‐pollination and sexual‐interference hypotheses have been tested individually before. However, despite the long‐standing interest in heterostyly – ever since Darwin's seminal work on primroses – the predictions derived from these two hypotheses have never been experimentally and systematically examined in the same system. Using distylous primroses (Primula elatior, P. vulgaris), we compare pollen transfer (i) between reciprocal and non‐reciprocal flowers; (ii) from anthers onto different parts of the pollinator's body; and (iii) within flower and between flowers of the same morph. We further test whether (iv) anther–stigma distance correlates with self‐pollen transfer and whether (v) seed set differs after pollinations with compatible, incompatible and both pollen types. Reciprocal herkogamy promotes differential placement of pollen onto different parts of the pollinator's body, thus effecting transfer of more pollen to reciprocal than to non‐reciprocal stigmas and realizing the key predictions of the disassortative‐pollination hypothesis. However, short‐styled flowers transfer pollen more disassortatively than long‐styled flowers in both species, whereas long‐styled flowers export more pollen to non‐reciprocal than to reciprocal stigmas in P. vulgaris, thus compromising male function in this species. Furthermore, larger distance between sexual organs lowers self‐ and intra‐morph pollination and the pollen incompatibility system decreases seed production after self‐pollination, thus diminishing sexual interference. Our results help us understand how the morphological and physiological components of heterostyly contribute to optimizing pollen transfer and minimizing self‐ and intra‐morph pollination, thus promoting more efficient outcrossing in species with this floral polymorphism.

Abstract Supergenes are nonrecombining genomic regions ensuring the coinheritance of multiple, coadapted genes. Despite the importance of supergenes in adaptation, little is known on how they originate. A classic example of supergene is the S locus controlling heterostyly, a floral heteromorphism occurring in 28 angiosperm families. In Primula, heterostyly is characterized by the cooccurrence of two complementary, self-incompatible floral morphs and is controlled by five genes clustered in the hemizygous, ca. 300-kb S locus. Here, we present the first chromosome-scale genome assembly of any heterostylous species, that of Primula veris (cowslip). By leveraging the high contiguity of the P. veris assembly and comparative genomic analyses, we demonstrated that the S-locus evolved via multiple, asynchronous gene duplications and independent gene translocations. Furthermore, we discovered a new whole-genome duplication in Ericales that is specific to the Primula lineage. We also propose a mechanism for the origin of S-locus hemizygosity via nonhomologous recombination involving the newly discovered two pairs of CFB genes flanking the S locus. Finally, we detected only weak signatures of degeneration in the S locus, as predicted for hemizygous supergenes. The present study provides a useful resource for future research addressing key questions on the evolution of supergenes in general and the S locus in particular: How do supergenes arise? What is the role of genome architecture in the evolution of complex adaptations? Is the molecular architecture of heterostyly supergenes across angiosperms similar to that of Primula?

Removal of internal transcribed spacer 2 (ITS2) from pre-ribosomal RNA is essential to make functional ribosomes. This complicated processing reaction begins with a single endonucleolytic cleavage followed by exonucleolytic trimming at both new cleavage sites to generate mature 5.8S and 25S rRNA. We reconstituted the 7S→5.8S processing branch within ITS2 using purified exosome and its nuclear cofactors. We find that both Rrp44’s ribonuclease activities are required for initial RNA shortening followed by hand over to the exonuclease Rrp6. During the in vitro reaction, ITS2-associated factors dissociate and the underlying ‘foot’ structure of the pre-60S particle is dismantled. 7S pre-rRNA processing is independent of 5S RNP rotation, but 26S→25S trimming is a precondition for subsequent 7S→5.8S processing. To complete the in vitro assay, we reconstituted the entire cycle of ITS2 removal with a total of 18 purified factors, catalysed by the integrated activities of the two participating RNA-processing machines, the Las1 complex and nuclear exosome. Excision of internal transcribed spacer 2 (ITS2) within eukaryotic pre-ribosomal RNA is essential for ribosome function. Here, the authors reconstitute the entire cycle of ITS2 processing in vitro using purified components, providing insights into the cleavage process and demonstrating that 26S pre-rRNA processing necessarily precedes 7S pre-rRNA processing.

The DEAD-box protein DBP5 is involved in yeast mRNA export, though the mechanism by which it helps to remodel and release transcripts on the cytoplasmic face of the nuclear pore complex has been unclear. The structures of DBP5 in complex with the mRNA and AMPPNP, as well as with the nucleoporin NUP214, indicate that the transcript and nucleoporin compete for the same binding site, suggesting a model for the sequence of events occurring at the last step of export. The DEAD-box protein DBP5 is essential for mRNA export in both yeast and humans. It binds RNA and is concentrated and locally activated at the cytoplasmic side of the nuclear pore complex. We have determined the crystal structures of human DBP5 bound to RNA and AMPPNP, and bound to the cytoplasmic nucleoporin NUP214. The structures reveal that binding of DBP5 to nucleic acid and to NUP214 is mutually exclusive. Using in vitro assays, we demonstrate that NUP214 decreases both the RNA binding and ATPase activities of DBP5. The interactions are mediated by conserved residues, implying a conserved recognition mechanism. These results suggest a framework for the consecutive steps leading to the release of mRNA at the final stages of nuclear export. More generally, they provide a paradigm for how binding of regulators can specifically inhibit DEAD-box proteins.

Uniparental reproduction is advantageous when lack of mates limits outcrossing opportunities in plants. Baker’s law predicts an enrichment of uniparental reproduction in habitats colonized via long-distance dispersal, such as volcanic islands. To test it, we analyzed reproductive traits at multiple hierarchical levels and compared seed-set after selfing and crossing experiments in both island and mainland populations of Limonium lobatum , a widespread species that Baker assumed to be self-incompatible because it had been described as pollen-stigma dimorphic, i.e., characterized by floral morphs differing in pollen-surface morphology and stigma-papillae shape that are typically self-incompatible. We discovered new types and combinations of pollen and stigma traits hitherto unknown in the literature on pollen-stigma dimorphism and a lack of correspondence between such combinations and pollen compatibility. Contrary to previous reports, we conclude that Limonium lobatum comprises both self-compatible and self-incompatible plants characterized by both known and previously undescribed combinations of reproductive traits. Most importantly, plants with novel combinations are overrepresented on islands, selfed seed-set is higher in islands than the mainland, and insular plants with novel pollen-stigma trait-combinations disproportionally contribute to uniparental reproduction on islands. Our results thus support Baker’s law, connecting research on reproductive and island biology.

The PI3K-related kinase (PIKK) SMG1 monitors the progression of metazoan nonsense-mediated mRNA decay (NMD) by phosphorylating the RNA helicase UPF1. Previous work has shown that the activity of SMG1 is impaired by small molecule inhibitors, is reduced by the SMG1 interactors SMG8 and SMG9, and is downregulated by the so-called SMG1 insertion domain. However, the molecular basis for this complex regulatory network has remained elusive. Here, we present cryo-electron microscopy reconstructions of human SMG1-9 and SMG1-8-9 complexes bound to either a SMG1 inhibitor or a non-hydrolyzable ATP analog at overall resolutions ranging from 2.8 to 3.6 Å. These structures reveal the basis with which a small molecule inhibitor preferentially targets SMG1 over other PIKKs. By comparison with our previously reported substrate-bound structure (Langer et al.,2020), we show that the SMG1 insertion domain can exert an autoinhibitory function by directly blocking the substrate-binding path as well as overall access to the SMG1 kinase active site. Together with biochemical analysis, our data indicate that SMG1 autoinhibition is stabilized by the presence of SMG8. Our results explain the specific inhibition of SMG1 by an ATP-competitive small molecule, provide insights into regulation of its kinase activity within the NMD pathway, and expand the understanding of PIKK regulatory mechanisms in general.