Cytoplasmic TAF2-TAF8-TAF10 complex provides evidence for nuclear holo-TFIID assembly from preformed submodules

General transcription factor TFIID is a cornerstone of RNA polymerase II transcription initiation in eukaryotic cells. How human TFIID-a megadalton-sized multiprotein complex composed of the TATA-binding protein (TBP) and 13 TBP-associated factors (TAFs)-assembles into a functional transcription factor is poorly understood. Here we describe a heterotrimeric TFIID subcomplex consisting of the TAF2, TAF8 and TAF10 proteins, which assembles in the cytoplasm. Using native mass spectrometry, we define the interactions between the TAFs and uncover a central role for TAF8 in nucleating the complex. X-ray crystallography reveals a non-canonical arrangement of the TAF8-TAF10 histone fold domains. TAF2 binds to multiple motifs within the TAF8 C-terminal region, and these interactions dictate TAF2 incorporation into a core-TFIID complex that exists in the nucleus. Our results provide evidence for a stepwise assembly pathway of nuclear holo-TFIID, regulated by nuclear import of preformed cytoplasmic submodules

Resolving crystallisation ages of Archean mafic-ultramafic rocks using the Re-Os isotope system

Rhenium-Osmium (Re-Os) isotope and elemental data are presented for mafic-ultramafic rocks from the central region of the Lewisian Archean terrain in northwest Scotland. These results give a best estimate for the time of emplacement of the mafic-ultramafic bodies of 2686.7 ± 14.7 Myr (2σ). The initial 187Os/188Os isotope ratio of 0.10940 ± 0.00076 indicates that such material possessed a chondritic Os isotope composition, which suggests that these rocks were formed by direct melting of mantle material, consistent with major and trace element constraints on their formation. Nevertheless, the Re-Os systematics of some of the mafic-ultramafic rocks in the Lewisian have been significantly disturbed, such that the original age information has been lost. These rocks lie on a regression line that defines an age of ~ 3260 Myr, and a negative initial Os isotope composition, suggesting perturbation of the Re-Os system, either through assimilation or post-emplacement elemental exchange. Such a process also appears to have affected the Sm-Nd systematics in the same samples. Crustal assimilation can account for the observed Os and Nd isotope variations but only if the assimilated material possessed 187Os/188Os values of ca. 25 at ~ 2687 Myr. In contrast, the surrounding gneisses and metasediments preserve present-day measured 187Os/188Os values of between 3 and 16. Rather, the spatial variation of initial Os and Nd isotope compositions suggests that isotope perturbation was caused by local sub-solidus element exchange between different lithologies, consistent with major element data and petrographic observations. Taken together, these results highlight the utility of the Re-Os isotope system for obtaining precise ages for Archean mafic-ultramafic rocks, and as a sensitive petrogenetic tracer capable of discriminating between assimilation or elemental exchange. (C) 2000 Elsevier Science B.V. All rights reserved

Correlated Os-Pb-Nd-Sr isotopes in the Austral-Cook chain basalts: the nature of mantle components in plume sources

Osmium (Os), strontium (Sr), neodymium (Nd) and lead (Pb) isotopes have been measured on a suite of aphyric basalts from 12 islands of the Austral-Cook island archipelago, an area which exhibits a range in Pb isotope compositions that encompasses almost the entire range displayed by ocean island basalts (OIB). Although the samples have Os concentrations (1.69-34.80 ppt) at the lower end of the range measured for OIB, they display a range of initial 187Os/ 188Os ratios (between 0.1279 and 0.1594) similar to that defined by olivine-phyric, Os-rich OIB. Positive Os-Nd, Os-Pb and negative Os-Sr isotope correlations are documented, indicating that the isotopic arrays represent various proportions of mixing between a HIMU-type end-member represented by Mangaia and Tubuai islands and characterized by radiogenic Os and Pb isotopic compositions, and an end-member represented by Rarotonga island which is characterized by unradiogenic Os and intermediate Sr, Nd and Pb isotopic compositions. The HIMU signature of the mantle component involved in Tubuai-Mangaia mantle sources requires long-term enrichments of U and Th relative to Pb and Re relative to Os, without associated increase in Rb/Sr, that are consistent with recycled oceanic crust. The end-member represented by Rarotonga basalts shows Os, Sr, Nd, and Pb isotopic signatures similar to those presumed for the 'bulk silicate earth' (BSE), which cannot be obtained by mixing the four mantle components (DMM, HIMU and EMI and 2) generally used to circumscribe the Sr-Nd-Pb isotopic data of OIB. The primitive-like isotopic characters of this end-member might be ascribed to the presence of undepleted material from a lower segment of the mantle in the source of the Austral-Cook island basalts (and more specifically Rarotonga basalts); however, such a hypothesis is challenged by both the absence of a primordial 3He signature and the non-primitive Ce/Pb and Nb/U values for the Austral-Cook island basalts. Alternatively, assuming that the primitive-like isotopic composition of the Rarotonga samples reflects mixing proportions between the HIMU component and a mantle component characterized by unradiogenic Os, Nd and Pb and radiogenic Sr isotopic composition relative to BSE, the involvement of recycled, old subcontinental lithosphere in the genesis of this component should then be considered. © 2001 Elsevier Science B.V. All rights reserved