The crystal structure of Escherichia coli glutaminyl-tRNA synthetase (GlnRS) complexed with its cognate glutaminyl transfer RNA (tRNA(Gln] and adenosine triphosphate (ATP) has been derived from a 2.8 angstrom resolution electron density map and the known protein and tRNA sequences. The 63.4-kilodalton monomeric enzyme consists of four domains arranged to give an elongated molecule with an axial ratio greater than 3 to 1. Its interactions with the tRNA extend from the anticodon to the acceptor stem along the entire inside of the L of the tRNA. The complexed tRNA retains the overall conformation of the yeast phenylalanine tRNA (tRNA(Phe] with two major differences: the 3' acceptor strand of tRNA(Gln) makes a hairpin turn toward the inside of the L, with the disruption of the final base pair of the acceptor stem, and the anticodon loop adopts a conformation not seen in any of the previously determined tRNA structures. Specific recognition elements identified so far include (i) enzyme contacts with the 2-amino groups of guanine via the tRNA minor groove in the acceptor stem at G2 and G3; (ii) interactions between the enzyme and the anticodon nucleotides; and (iii) the ability of the nucleotides G73 and U1.A72 of the cognate tRNA to assume a conformation stabilized by the protein at a lower free energy cost than noncognate sequences. The central domain of this synthetase binds ATP, glutamine, and the acceptor end of the tRNA as well as making specific interactions with the acceptor stem.2+t is

The anticancer activity of cis-diamminedichloroplatinum(II) (cisplatin) arises from its ability to damage DNA, with the major adducts formed being intrastrand d(GpG) and d(ApG) crosslinks. These crosslinks bend and unwind the duplex, and the altered structure attracts high-mobility-group domain (HMG) and other proteins. This binding of HMG-domain proteins to cisplatin-modified DNA has been postulated to mediate the antitumour properties of the drug. Many HMG-domain proteins recognize altered DNA structures such as four-way junctions and cisplatin-modified DNA, but until now the molecular basis for this recognition was unknown. Here we describe mutagenesis, hydroxyl-radical footprinting and X-ray studies that elucidate the structure of a 1:1 cisplatin-modified DNA/HMG-domain complex. Domain A of the structure-specific HMG-domain protein HMG1 binds to the widened minor groove of a 16-base-pair DNA duplex containing a site-specific cis-[Pt(NH3)2[d(GpG)-N7(1),-N7(2)]] adduct. The DNA is strongly kinked at a hydrophobic notch created at the platinum-DNA crosslink and protein binding extends exclusively to the 3' side of the platinated strand. A phenylalanine residue at position 37 intercalates into a hydrophobic notch created at the platinum crosslinked d(GpG) site and binding of the domain is dramatically reduced in a mutant in which alanine is substituted for phenylalanine at this position.

Our 1.6 A structure should provide an excellent framework for analyzing the effects of Zif268 mutations, for modeling related zinc finger-DNA complexes, and for designing and selecting Zif268 variants that will recognize other DNA sites.

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The editing enzyme double-stranded RNA adenosine deaminase includes a DNA binding domain, Zalpha, which is specific for left-handed Z-DNA. The 2.1 angstrom crystal structure of Zalpha complexed to DNA reveals that the substrate is in the left-handed Z conformation. The contacts between Zalpha and Z-DNA are made primarily with the "zigzag" sugar-phosphate backbone, which provides a basis for the specificity for the Z conformation. A single base contact is observed to guanine in the syn conformation, characteristic of Z-DNA. Intriguingly, the helix-turn-helix motif, frequently used to recognize B-DNA, is used by Zalpha to contact Z-DNA.

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The 2.5 A resolution structure of a cocrystal containing the paired domain from the Drosophila paired (prd) protein and a 15 bp site shows structurally independent N-terminal and C-terminal subdomains. Each of these domains contains a helical region resembling the homeodomain and the Hin recombinase. The N-terminal domain makes extensive DNA contacts, using a novel beta turn motif that binds in the minor groove and a helix-turn-helix unit with a docking arrangement surprisingly similar to that of the lambda repressor. The C-terminal domain is not essential for prd binding and does not contact the optimized site. All known developmental missense mutations in the paired box of mammalian Pax genes map to the N-terminal subdomain, and most of them are found at the protein-DNA interface.

The refined crystal structure of Escherichia coli glutaminyl transfer RNA synthetase complexed with transfer RNA(Gln) and ATP reveals that the structure of the anticodon loop of the enzyme-bound tRNA(Gln) differs extensively from that of the known crystal structures of uncomplexed tRNA molecules. The anticodon stem is extended by two non-Watson-Crick base pairs, leaving the three anti-codon bases unpaired and splayed out to bind snugly into three separate complementary pockets in the protein. These interactions suggest that the entire anticodon loop provides essential sites for glutaminyl tRNA synthetase discrimination among tRNA molecules.