Is tRNA binding or tRNA mimicry mandatory for translation factors?

tRNA is the adaptor in the translation process. The ribosome has three sites for tRNA, the A-, P-, and E-sites. The tRNAs bridge between the ribosomal subunits with the decoding site and the mRNA on the small or 30S subunit and the peptidyl transfer site on the large or 50S subunit. The possibility that translation release factors could mimic tRNA has been discussed for a long time, since their function is very similar to that of tRNA. They identify stop codons of the mRNA presented in the decoding site and hydrolyse the nascent peptide from the peptidyl tRNA in the peptidyl transfer site. The structures of eubacterial release factors are not yet known, and the first example of tRNA mimicry was discovered when elongation factor G (EF-G) was found to have a closely similar shape to a complex of elongation factor Tu (EF-Tu) with aminoacyl-tRNA. An even closer imitation of the tRNA shape is seen in ribosome recycling factor (RRF). The number of proteins mimicking tRNA is rapidly increasing. This primarily concerns translation factors. It is now evident that in some sense they are either tRNA mimics, GTPases or possibly both.     

Hall-velocity limited magnetoconductivity in a 2D Wigner solid

The magnetoconductivity σ(B) of a classical 2D electron crystal on superfluid⁴He is non-linear. Experimentally we find a contribution to σ(B) which at constant field, gives σ(B)∞J _x, the current density, while at constant current, σ(B) ∞ 1/B. In this region the Hall velocity ν_H slowly approaches the ripplon velocity ν_I at the first reciprocal lattice vector, due to strong electron-ripplon interactions with the helium. The magnetoconductivity decreases sharply for ν_H>ν_I. Fluctuations in σ(B) are seen above the melting temperature.     

On conditions for polyconvexity

We give an example of a smooth function , which is not polyconvex and which has the property that its restriction to any ball of radius one can be extended to a smooth polyconvex function . In particular, it implies that there exists no `local condition' which is necessary and sufficient for polyconvexity of functions , where , . We also briefly discuss connections with quasiconvexity.

     

A local framework for calculating coupled cluster singles and doubles excitation energies (LoFEx-CCSD)

ABSTRACT

The recently developed Local Framework for calculating Excitation energies (LoFEx) is extended to the coupled cluster singles and doubles (CCSD) model. In the new scheme, a standard CCSD excitation energy calculation is carried out within a reduced excitation orbital space (XOS), which is composed of localised molecular orbitals and natural transition orbitals determined from time-dependent Hartree–Fock theory. The presented algorithm uses a series of reduced second-order approximate coupled cluster singles and doubles (CC2) calculations to optimise the XOS in a black-box manner. This ensures that the requested CCSD excitation energies have been determined to a predefined accuracy compared to a conventional CCSD calculation. We present numerical LoFEx-CCSD results for a set of medium-sized organic molecules, which illustrate the black-box nature of the approach and the computational savings obtained for transitions that are local compared to the size of the molecule. In fact, for such local transitions, the LoFEx-CCSD scheme can be applied to molecular systems where a conventional CCSD implementation is intractable.     

Regularity of quasiconvex envelopes

We prove that the quasiconvex envelope of a differentiable function which satisfies natural growth conditions at infinity is a function. Without the growth conditions the result fails in general. We also obtain results on higher regularity (in the sense of ) and similar results for other types of envelopes, including polyconvex and rank-1 convex envelopes. Received January 11, 2000/ Accepted January 14, 2000 / Published online June 28, 2000     

Second-harmonic generation spectroscopy on quantum wells: Au on Si(111)

×):Au-structure prior to film growth leads to increased amplitude in the oscillations indicating improved growth characteristics compared to the 7×7 substrate. The position of the maxima shifts towards lower coverage and the oscillation period decreases with increasing pump frequency. This behaviour is an unambiguous signature of quantum well resonances and, hence, clearly demonstrates that Au deposition on Si(111)(×):Au leads to well-defined quantum well structures with atomically flat interfaces. Furthermore, the rotationally anisotropic contribution to SHG also shows coverage oscillations as a result of quantum well effects and demonstrates the presence of structural order. Received: 12 October 1998     

A Method for the Selection of Catalytic Activity Using Phage Display and Proximity Coupling

Phage display has been used extensively for the selection of proteins with binding sites for ligands. Here, as illustrated with the example of DNA polymerase, the use of phage display for selection according to catalytic activity is described. Active enzymes are selected by binding of the reaction product P (see the scheme) cross-linked in the proximity of the enzyme E that catalyzed the reaction with the substrate S.     

Partial Regularity of Strong Local Minimizers in the Multi-Dimensional Calculus of Variations

Let ⁿ be a bounded domain and F: a given strongly quasiconvex integrand of class C ² satisfying the growth condition for some c>0 and 2p<. Consider the multiple integral where uW ^1,p (, ^N ). The main result of the paper is the proof that any strong local minimizer of I[·] is of class C ^1, _loc for any (0,1) on an open set of full n-dimensional measure. In the case of weak local minimizers we establish the same result under the extra assumption that the oscillations in the gradient of the minimizer are not too large. Without such an assumption weak local minimizers need not be partially regular as we show by a class of examples. We also briefly discuss the question of existence of strong local minimizers for I[·] and connections of our results to extensions of Weierstrass sufficiency theorem to the multi-dimensional setting.