Bovine seminal ribonuclease, the only dimeric ribonuclease described thus far, is found to exist in two different quaternary structure forms. In one, the N-terminal segment (residues 1-17) of each subunit is interchanged with the remaining segment of the other subunit, whereas in the second, such interchange does not occur. Functionally, they differ in that the catalytic activity of the form with interchange can be modulated by the substrate, whereas the noninterchange form exhibits no cooperativity. Each form can convert into the other, up to an equilibrium ratio, which is that found for the isolated protein. The results of refolding experiments of unfolded protein chains suggest that also in vivo the form lacking interchange may be produced first and is then partially transformed into the other dimeric form until equilibrium is reached. Although the implications of these findings may not be immediately apparent, they are intriguing and may have an impact on the unusual noncatalytic actions of the protein, such as its selective cytotoxicity toward tumor cells, activated T cells, and differentiated male germ cells.Bovine seminal ribonuclease (BS-RNase) is a homodimeric ribonuclease-in fact, the only dimeric RNase isolated thus far. Its enzymic properties are unusual for a ribonuclease, as it cleaves effectively single-and double-stranded RNA and is allosterically regulated in the rate-determining step of the reaction. These properties are matched by its unusual biological, noncatalytic actions, including its selective toxicity toward tumor cells, activated T cells, and the male germ cell line (ref. 1 and references cited therein).In the structure of naturally dimeric BS-RNase determined by x-ray crystallography (2, 3), the two subunits interchange their N-terminal segments (see Fig. 1A) as do the monomers of artificially dimerized bovin' pancreatic RNase (RNase A) in the structure proposed by Crestfield and others (4, 5). A significant consequeoce of ths 'structural arrangement is the composite nature 9f the active sites of dimerized RNase A and of naturally dimeric BS-RNase. Composite, shared active sites have been previously found for a few oligomeric enzymes (6-9). However, in all of these cases, dissociation of the oligomers produced inactive monomers. In contrast, isolated BS-RNase monomers are active-in fact, more active than the parent dimeric enzyme, although they lack its typical allosteric properties (10).The dimeric structure of BS-RNase is maintained not only by noncovalent interactions between subunits (3) but also by two intersubunit disulfides, which link Cys-31 and Cys-32 of one subunit with Cys-32 and Cys-31, respectively, of the other subunit (11, 12). We have previously observed (12) that selective reduction of the intersubunit disulfides monomerizes only a fraction of the dimeric protein. This observation, which had not been fully understood thus far, has since been confirmed repeatedly, also in other laboratories (13), with protein preparations obtained with different purification ...

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The Ets family of transcription factors, of which there are now about 35 members regulate gene expression during growth and development. They share a conserved domain of around 85 amino acids which binds as a monomer to the DNA sequence 5'-C/AGGAA/T-3'. We have determined the crystal structure of an ETS domain complexed with DNA, at 2.3-A resolution. The domain is similar to alpha + beta (winged) 'helix-turn-helix' proteins and interacts with a ten-base-pair region of duplex DNA which takes up a uniform curve of 8 degrees. The domain contacts the DNA by a novel loop-helix-loop architecture. Four of amino acids that directly interact with the DNA are highly conserved: two arginines from the recognition helix lying in the major groove, one lysine from the 'wing' that binds upstream of the core GGAA sequence, and another lysine, from the 'turn' of the 'helix-turn-helix' motif, which binds downstream and on the opposite strand.

NMR, molecular dynamics and mechanics calculations, and CD spectroscopy were used to characterise three tetramolecular quadruplex complexes: [d(TG(Br)GGT)](4), [d(TGG(Br)GT)](4) and [d(TGGG(Br)T)](4), where G(Br) indicates an 8-bromoguanine residue. All three quadruplexes are characterised by a 4-fold symmetry with all strands parallel to each other and, differently to what has been observed for other parallel quadruplex structures, with a tetrad (formed by 8-Br-dGs) in a syn conformation. The whole of the data demonstrates that the replacement in turn of different dG residues with 8-Br-dG in the sequence 5[prime or minute]-TGGGT-3[prime or minute] affects the resulting structures in different ways, leading to different CD profiles and thermal stabilities. Particularly, [d(TG(Br)GGT)](4) and [d(TGG(Br)GT)](4) are more stable than the unmodified sequence, whereas [d(TGGG(Br)T)](4) is much less stable than the natural counterpart. The conformational features found in the three quadruplexes might, in principle, amplify the range of applicability of synthetic oligonucleotides as aptamers or catalysts, by providing novel structural motifs with different molecular recognition capabilities from those of native DNA sequences.

Among non-canonical DNA secondary structures, G-quadruplexes are currently widely studied because of their probable involvement in many pivotal biological roles, and for their potential use in nanotechnology. The overall quadruplex scaffold can exhibit several morphologies through intramolecular or intermolecular organization of G-rich oligodeoxyribonucleic acid strands. In particular, several G-rich strands can form higher order assemblies by multimerization between several G-quadruplex units. Here, we report on the identification of a novel dimerization pathway. Our Nuclear magnetic resonance, circular dichroism, UV, gel electrophoresis and mass spectrometry studies on the DNA sequence dCGGTGGT demonstrate that this sequence forms an octamer when annealed in presence of K+ or NH4+ ions, through the 5′-5′ stacking of two tetramolecular G-quadruplex subunits via unusual G(:C):G(:C):G(:C):G(:C) octads.