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1.Crystal structure of a bacteriophage T7 DNA replication complex at 2.2A° resolution

Sylvie Doublie´ , Stanley Tabor, Alexander M. Long, Charles C. Richardson & Tom Ellenberger

Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, 240 Longwood Avenue, Boston, Massachusetts 02115, USA
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DNA polymerases change their specificity for nucleotide substrates with each catalytic cycle, while achieving error frequencies in the range of 10-5 to 10-6. Here we present a 2.2A° crystal structure of the replicative DNA polymerase from bacteriophage T7 complexed with a primer–template and a nucleoside triphosphate in the polymerase active site. The structure illustrates how nucleotides are selected in a template-directed manner, and provides a structural basis for a metal-assisted mechanism of phosphoryl transfer by a large group of related polymerases.
£¨NATURE |VOL 391 | 15 JANUARY 1998£©

2. Crystal structure of the yeast MATα2/MCM1/DNA ternary complex

Song Tan & Timothy J. Richmond
ETH-Zu¨rich, Institut fu¨r Molekularbiologie und Biophysik, ETH-Ho¨nggerberg, CH-8093 Zu¨rich, Switzerland
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The structure of a complex containing the homeodomain repressor protein MATα2 and the MADS-box transcription factor MCM1 bound to DNA has been determined by X-ray crystallography at 2.25A° resolution. It reveals the protein–protein interactions responsible for cooperative binding of MATα2 and MCM1 to DNA. The otherwise flexible amino-terminal extension of the MATα2 homeodomain forms a b-hairpin that grips the MCM1 surface through parallel β-strand hydrogen bonds and close-packed, predominantly hydrophobic, side chains. DNA bending induced by MCM1 brings the two proteins closer together, facilitating their interaction. An unusual feature of the complex is that an eight-amino-acid sequence adopts an a-helical conformation in one of two copies of the MATα2 monomer and a β-strand conformation in the other. This ‘chameleon’ sequence of MATα2 may be important for recognizing natural operator sites.
£¨NATURE | VOL 391 | 12 FEBRUARY 1998£©

3.Crystal structure of the spliceosomal U2B‘’–U2A‘ protein complex bound to a fragment of U2 small nuclear RNA

Stephen R. Price*, Philip R. Evans & Kiyoshi Nagai
MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK
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We have determined the crystal structure at 2.4A° resolution of a ternary complex between the spliceosomal U2B’‘/U2A’ protein complex and hairpin-loop IV of U2 small nuclear RNA. Unlike its close homologue the U1A protein, U2B‘’ binds to its cognate RNA only in the presence of U2A‘, which contains leucine-rich repeats in its sequence. The concave surface of a parallel b-sheet within the leucine-rich-repeat region of U2A‘ interacts with the ribonucleoprotein domain of U2B‘’ on the surface opposite its RNA-binding surface. The basic carboxy-terminal region of U2A’ interacts with the RNA stem. The crystal structure reveals how protein–protein interaction regulates RNA-binding specificity, and how replacing only a few key residues allows the U2B’‘ and U1A proteins to discriminate between their cognate RNA hairpins by forming alternative networks of interactions.
£¨NATURE |VOL 394 | 13 AUGUST 1998£©

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