2-Aminopurine flipped into the active site of the adenine-specific DNA methyltransferase M.TaqI: crystal structures and time-resolved fluorescence

We report the crystal structure of the DNA adenine-N6 methyltransferase, M.TaqI, complexed with DNA, showing the fluorescent adenine analog, 2-aminopurine, flipped out of the DNA helix and occupying virtually the same position in the active site as the natural target adenine. Time-resolved fluorescence spectroscopy of the crystalline complex faithfully reports this state: base flipping is accompanied by the loss of the very short ( approximately 50 ps) lifetime component associated with fully base-stacked 2-aminopurine in DNA, and 2-aminopurine is subject to considerable quenching by pi-stacking interactions with Tyr108 in the catalytic motif IV (NPPY). This proves 2-aminopurine to be an excellent probe for studying base flipping by M.TaqI and suggests similar quenching in the active sites of DNA and RNA adenine-N6 as well as DNA cytosine-N4 methyltransferases sharing the conserved motif IV. In solution, the same distinctive fluorescence response confirms complete destacking from DNA and is also observed when the proposed key residue for base flipping by M.TaqI, the target base partner thymine, is substituted by an abasic site analog. The corresponding cocrystal structure shows 2-aminopurine in the active site of M.TaqI, demonstrating that the partner thymine is not essential for base flipping. However, in this structure, a shift of the 3' neighbor of the target base into the vacancy left after base flipping is observed, apparently replicating a stabilizing role of the missing partner thymine. Time-resolved fluorescence and acrylamide quenching measurements of M.TaqI complexes in solution provide evidence for an alternative binding site for the extra-helical target base within M.TaqI and suggest that the partner thymine assists in delivering the target base into the active site.     

On the crystalline cohomology of Deligne–Lusztig varieties

Let $X\to Y^0$ be an abelian prime-to-p Galois covering of smooth schemes over a perfect field k of characteristic $p>0$. Let Y be a smooth compactification of $Y^0$ such that $Y-Y^0$ is a normal crossings divisor on Y. We describe a logarithmic F-crystal on Y whose rational crystalline cohomology is the rigid cohomology of X, in particular provides a natural $W[F]$-lattice inside the latter; here W is the Witt vector ring of k. If a finite group G acts compatibly on X, $Y^0$ and Y then our construction is G-equivariant. As an example we apply it to Deligne–Lusztig varieties. For a finite field k, if $\mathbb{G}$ is a connected reductive algebraic group defined over k and $\mathbb{L}$ a k-rational torus satisfying a certain standard condition, we obtain a meaningful equivariant $W[F]$-lattice in the cohomology (ℓ-adic or rigid) of the corresponding Deligne–Lusztig variety and an expression of its reduction modulo p in terms of equivariant Hodge cohomology groups.     

Equivariant crystalline cohomology and base change

Given a perfect field of characteristic , a smooth proper -scheme , a crystal on relative to and a finite group acting on and , we show that, viewed as a virtual -module, the reduction modulo of the crystalline cohomology of is the de Rham cohomology of modulo . On the way we prove a base change theorem for the virtual -representations associated with -equivariant objects in the derived category of -modules.

     

Dilepton production from ρ-mesons in a quark-gluon plasma

Assuming that ρ-mesons exist in a quark-gluon plasma at temperatures close to the QCD phase transition, we calculate the dilepton production rate from q-ˉq annihilation via a ρ-meson state using Vector Meson Dominance. The result is compared to the rates from direct q-ˉq annihilation and from π⁺-π⁻ annihilation. Furthermore we discuss the suppression of low mass dileptons if the quarks assume an effective mass in the quark-gluon plasma. Received: 8 September 1999 / Revised version: 22 October 1999     

Mellin and Green symbols for boundary value problems on manifolds with edges

We introduce the algebra of smoothing Mellin and Green symbols in a pseudodifferential calculus for manifolds with edges. In addition, we define scales of weighted Sobolev spaces with asymptotics based on the Mellin transform and analyze the mapping properties of the operators on these spaces. This will allow us to obtain complete information on the regularity and asymptotics of solutions to elliptic equations on these spaces.     

Leontovich relations in thermal field theory

The application of generalized Kramers-Kronig relations, the so-called Leontovich relations, to thermal field theory is discussed. Medium effects contained in the full, thermal propagators can easily be taken into account by this method. As examples the collisional energy loss of a charged particle in a relativistic plasma and the radiation of energetic photons from a quark-gluon plasma are considered. Within the leading logarithmic approximation the results based on the hard thermal loop resummation technique are reproduced easily. However, the method presented here is more general and provides exact expressions, which allow in principle non-perturbative calculations. Received: 17 April 2000 / Published online: 6 July 2000     

Darstellung,Eigenschaften und Molekülstrukturen von Dimethylaminomethylferrocenyl‐Verbindungen ausgewählter Elemente der Gruppen 13 und 14

Preparation, Properties, and Molecular Structures of Dimethylaminomethyl Ferrocenyl Compounds of selected Elements of Group 13 and 14 Dimethylmetalchlorides of gallium and indium react with dimethylaminomethylferrocenyllithium (FcNLi) to give the corresponding dimethylmetaldimethylaminomethylferrocenes 1 and 2 [Me₂MFcN; M=Ga, In]. In a similar manner dialkylmetaldichlorides of germanium and tin yield the expected chlordialkylmetaldimethylaminomethylferrocenes 3 – 5 [R₂(Cl)MFcN; M=Ge; R = Me ( 3 ), M=Sn; R=Me ( 4 ), Ph ( 5 )]. In a reaction of Me₃Al and Me₂AlCl with dimethylaminomethylferrocene the formation of the 1 : 1 adducts 7 and 8 could be observed. All compounds were characterised by ¹H and ¹³C nmr spectroscopy. The molecular structures of 1 , 3 , 4 and 7 were determined. 3 and 4 build in contrast to 1 monomeric molecules with chelat rings as a result of the M–N coordination. Compound 7 consist of monomeric molecules with 4 coordinated Al atoms.     

Coupling of a Nucleoside with DNA by a Methyltransferase

How to outwit a methyltransferase: Methyltransferases (Mtases) catalyze the transfer of the activated methyl group from the cofactor S-adenosyl-L -methionine ( 1 ) to acceptors R within a large variety of biomolecules. Through the use of the cofactor analogue 2 a whole nucleoside was coupled to DNA in a Mtase-catalyzed reaction.