We report two new structures of the quadruplex d(TGGGGT)4 obtained by single crystal X-ray diffraction. In one of them a thymine tetrad is found. Thus the yeast telomere sequences d(TG1-3) might be able to form continuous quadruplex structures, involving both guanine and thymine tetrads. Our study also shows substantial differences in the arrangement of thymines when compared with previous studies. We find five different types of organization: (i) groove binding with hydrogen bonds to guanines from a neighbour quadruplex; (ii) partially ordered groove binding, without any hydrogen bond; (iii) stacked thymine triads, formed at the 3'ends of the quadruplexes; (iv) a thymine tetrad between two guanine tetrads. Thymines are stabilized in pairs by single hydrogen bonds. A central sodium ion interacts with two thymines and contributes to the tetrad structure. (v) Completely disordered thymines which do not show any clear location in the crystal. The tetrads are stabilized by either Na+ or Tl+ ions. We show that by using MAD methods, Tl+ can be unambiguously located and distinguished from Na+. We can thus determine the preference for either ion in each ionic site of the structure under the conditions used by us.

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The Py-crystallins form a superfamily of eye lens proteins comprised of multiple Greek motifs that are symmetrically organized into domains and higher assemblies. In the PB2-crystallin dimer each polypeptide folds into two similar domains that are related to monomeric y-crystallin by domain swapping. The crystal structure of the circularly permuted two-domain PB2 polypeptide shows that permutation converts intermolecular domain pairing into intramolecular pairing. However, the dimeric permuted protein is, in fact, half a native tetramer. This result shows how the sequential order of domains in multi-domain proteins can affect quaternary domain assembly.

The crystal structure of bovine lens gammaIIIb-crystallin at 2.5 A resolution previously reported was interpreted using a consensus sequence derived from related vertebrate sequences on the assumption that gammaIIIb-crystallin derived from the gammaC-crystallin gene. It has recently been shown that gammaIIIb is a product of the bovine gammaD gene. The structure of gammaIIIb has now been refined with the bovine gammaD sequence using new 1.95 A resolution synchrotron data. The crystallographic R factor was 20.4% for all 33 104 reflection data between 8.0 and 1.95 A measured at 277(1) K. The electron density fully supported the assignment of the gammaD sequence to gammaIIIb. The crystal belongs to space group P2(1)2(1)2(1) with two molecules of molecular mass 20 749 Da in the asymmetric unit in which 219 water molecules were located. The two-domain four-Greek-key motif highly symmetrical protein is very similar in structure to gammaB-crystallin (81% sequence identity). There is a single amino-acid deletion in gammaD in the linker region connecting the two domains. The intermolecular oganization in the crystal lattice is quite different from gammaB as a result of key mutations involving surface residues Leu51, Ile103 and His155. These point mutations will contribute to the intermolecular behaviour of the gamma-crystallins in the eye lens, where they are major components of the densely packed, high refractive index regions of the lens.

The deoxyoligonucleotide d(CGCAATTGCG) has previously been crystallized in four different space groups. The crystals diffract to moderate resolution (2.3±2.9 A Ê ). Here, a ®fth crystal form that diffracts to higher resolution (1.6 A Ê ) is presented which was obtained thanks to the use of Co 2+ and cryogenic temperatures. The availability of ®ve different crystal structures allows a thorough analysis of the conformational variability of this DNA sequence. It is concluded that the central hexamer sequence CAATTG has a practically constant conformation under all conditions, whilst the terminal base pairs at both ends vary considerably as a result of differing interactions in the crystals. The new crystal structure presented here is stabilized by guanine±Co 2+ ± guanine interactions and the formation of C1 + ±G8ÁC3 triplexes between neighbouring duplexes. As a result of the higher resolution of the crystal structure, a more regular structure was obtained and a clear de®nition of the spine of hydration was observed which was not visible in the four previous structures.

The crystal structures of the hexamer duplex d(CGTACG)(2) complexed with the intercalating anthraquinone derivative 1,5-bis[3-(diethylamino)propionamido]anthracene-9,10-dione and the acridine derivative 9-acridinyl tetralysine have been solved at 2.0- and 1.4-A resolution, respectively. In both cases, the drugs adopt multiple orientations within a large DNA cavity constituted by two groups of four approximately coplanar bases. Cations play a pivotal role in the crystal structure. Both complexes crystallise in the presence of Co(2+), Ba(2+) and Na(+) ions. They reveal at least two different types of coordination environments: (1) specific sites for Co(2+) interacting with N7 of guanine; (2) a central ionic site formed by four phosphate groups, which can be occupied by different ions. One more ionic site that is not always occupied by ions is also visible in the electron density map. All of them play a role in the crystal structure.

PDB Reference: anthraquinone-DNA complex, 3gdd.NDB Reference: anthraquinone-DNA complex, DD0106.The crystal structure of the telomeric sequence d(U Br AGG) interacting with an anthraquinone derivative has been solved by MAD. In all previously studied complexes of intercalating drugs, the drug is usually sandwiched between two DNA base pairs. Instead, the present structure looks like a crystal of stacked anthraquinone molecules in which isolated base pairs are intercalated. Unusual base pairs are present in the structure, such as GÁG and AÁU Br reverse WatsonCrick base pairs.

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