Transcription initiation by eukaryotic RNA polymerase (Pol) III relies on the TFIIE-related subcomplex C82/34/31. Here we combine crosslinking and hydroxyl radical probing to position the C82/34/31 subcomplex around the Pol III active center cleft. The extended winged helix (WH) domains 1 and 4 of C82 localize to the polymerase domains clamp head and clamp core, respectively, and the two WH domains of C34 span the polymerase cleft from the coiled-coil region of the clamp to the protrusion. The WH domains of C82 and C34 apparently cooperate with other mobile regions flanking the cleft during promoter DNA binding, opening, and loading. Together with published data, our results complete the subunit architecture of Pol III and indicate that all TFIIE-related components of eukaryotic and archaeal transcription systems adopt an evolutionarily conserved location in the upper part of the cleft that supports their functions in open promoter complex formation and stabilization. R NA polymerase III (Pol III) is the largest eukaryotic RNA polymerase, composed of 17 subunits with a total molecular weight of ∼0.7 MDa (1). Pol III synthesizes certain small untranslated RNAs (e.g., tRNAs, 5S rRNA, U6 snRNA, and 7SL RNA) involved in RNA processing and translation and in protein translocation (2, 3). Human Pol III mutations have been implicated in a neurodegenerative disorder, hypomyelinating leukodystrophy (4-6).The three eukaryotic RNA polymerases share a similar 12-subunit core as represented by the X-ray structure of yeast Pol II (1). In addition to the core, Pol III contains five specific subunits forming two subcomplexes, C37/53 and C82/34/31. The C37/53 subcomplex participates in promoter opening, transcription termination, and polymerase reinitiation (7-9). The C34 subunit of the C82/34/31 subcomplex plays a role in open complex formation and in recruiting Pol III to the preinitiation complex (PIC) through interaction with TFIIB-related factor 1 (Brf1) (10-13). The human RPC62/39/32 subcomplex, homologous to the yeast C82/34/31 complex, is dissociable and required for promoter-specific initiation (11).The two Pol III-specific subcomplexes contain structural domains homologous to domains in the Pol II transcription factors TFIIF and TFIIE, including the TFIIF-related dimerization module in the C37/53 subcomplex and several TFIIE-related winged helix (WH) domains in subunits C82 and C34 (14-20). TFIIE is composed of two subunits, Tfa1 and Tfa2, in yeast, or TFIIEα and TFIIEβ in humans. Whereas Tfa1 bears an extended WH (eWH) domain in its N-terminal region, Tfa2 has two adjacent WH domains (21,22). Two adjacent Tfa2-related WH domains are also present in the C34 subunit and its human homolog RPC39. Pol I contains the A49/34.5 subcomplex that features a TFIIF-like dimerization module and a tandem WH domain that contains two Tfa2-like WH folds in the C-terminal region of the A49 subunit (18). The crystal structure of the human C82 homolog RPC62 contains four Tfa1-like eWH domains (eWH1-4) that are structurally organized aroun...

Evolutionary origin of muscle is a central question when discussing mesoderm evolution. Developmental mechanisms underlying somatic muscle development have mostly been studied in vertebrates and fly where multiple signals and hierarchic genetic regulatory cascades selectively specify myoblasts from a pool of naive mesodermal progenitors. However, due to the increased organismic complexity and distant phylogenetic position of the two systems, a general mechanistic understanding of myogenesis is still lacking. In this study, we propose a gene regulatory network (GRN) model that promotes myogenesis in the sea urchin embryo, an early branching deuterostome. A fibroblast growth factor signaling and four Forkhead transcription factors consist the central part of our model and appear to orchestrate the myogenic process. The topological properties of the network reveal dense gene interwiring and a multilevel transcriptional regulation of conserved and novel myogenic genes. Finally, the comparison of the myogenic network architecture among different animal groups highlights the evolutionary plasticity of developmental GRNs.DOI: http://dx.doi.org/10.7554/eLife.07343.001

Hypoxia signaling is an ancient pathway by which animals can respond to low oxygen. Malfunction of this pathway disturbs hypoxic acclimation and can result in various diseases, including cancers. The role of hypoxia signaling in early embryogenesis remains unclear. Here, we show that in the blastula of the sea urchin , hypoxia-inducible factor α (HIFα), the downstream transcription factor of the hypoxia pathway, is localized and transcriptionally active on the future dorsal side. This asymmetric distribution is attributable to its oxygen-sensing ability. Manipulations of the HIFα level entrained the dorsoventral axis, as the side with the higher level of HIFα tends to develop into the dorsal side. Gene expression analyses revealed that HIFα restricts the expression of to the ventral side and activates several genes encoding transcription factors on the dorsal side. We also observed that intrinsic hypoxic signals in the early embryos formed a gradient, which was disrupted under hypoxic conditions. Our results reveal an unprecedented role of the hypoxia pathway in animal development.

The spatially opposed expression of Antidorsalizing morphogenetic protein (Admp) and BMP signals controls dorsoventral (DV) polarity across Bilateria and hence represents an ancient regulatory circuit. Here, we show that in addition to the conserved admp1 that constitutes the ancient circuit, a second admp gene (admp2) is present in Ambulacraria (Echinodermata+Hemichordata) and two marine worms belonging to Xenoturbellida and Acoelomorpha. The phylogenetic distribution implies that the two admp genes were duplicated in the Bilaterian common ancestor and admp2 was subsequently lost in chordates and protostomes. We show that the ambulacrarian admp1 and admp2 are under opposite transcriptional control by BMP signals and knockdown of Admps in sea urchins impaired their DV polarity. Over-expression of either Admps reinforced BMP signaling but resulted in different phenotypes in the sea urchin embryo. Our study provides an excellent example of signaling circuit rewiring and protein functional changes after gene duplications.

Evolutionary origin of muscle is a central question when discussing mesoderm evolution.Developmental mechanisms underlying somatic muscle development have mostly been studied in vertebrates and fly where multiple signals and hierarchic genetic regulatory cascades selectively specify myoblasts from a pool of naive mesodermal progenitors. However, due to the increased organismic complexity and distant phylogenetic position of the two systems, a general mechanistic understanding of myogenesis is still lacking. In this study, we propose a gene regulatory network (GRN) model that promotes myogenesis in the sea urchin embryo, an early branching deuterostome. A fibroblast growth factor signaling and four Forkhead transcription factors consist the central part of our model and appear to orchestrate the myogenic process. The topological properties of the network reveal dense gene interwiring and a multilevel transcriptional regulation of conserved and novel myogenic genes. Finally, the comparison of the myogenic network architecture among different animal groups highlights the evolutionary plasticity of developmental GRNs.