A. Sureau

The Polypyrimidine Tract Binding Protein (PTB) Represses Splicing of Exon 6B from the β-Tropomyosin Pre-mRNA by Directly Interfering with the Binding of the U2AF65 Subunit

ABSTRACT Splicing of exon 6B from the β-tropomyosin pre-mRNA is repressed in nonmuscle cells and myoblasts by a complex array of intronic elements surrounding the exon. In this study, we analyzed the proteins that mediate splicing repression of exon 6B through binding to the upstream element. We identified the polypyrimidine tract binding protein (PTB) as a component of complexes isolated from myoblasts that assemble onto the branch point region and the pyrimidine tract. In vitro splicing assays and PTB knockdown experiments by RNA interference demonstrated that PTB acts as a repressor of splicing of exon 6B. Using psoralen experiments, we showed that PTB acts at an early stage of spliceosome assembly by preventing the binding of U2 snRNA on the branch point. Using UV cross-linking and immunoprecipitation experiments with site-specific labeled RNA in PTB-depleted nuclear extracts, we found that the decrease in PTB was correlated with an increase in U2AF65. In addition, competition experiments showed that PTB is able to displace the binding of U2AF65 on the polypyrimidine tract. Our results strongly support a model whereby PTB competes with U2AF65 for binding to the polypyrimidine tract.

Soft-x-ray amplification by lithium like ions in recombining hot plasmas

This paper describes calculations and experiments about soft-x-ray amplification by lithiumlike ions in recombining laser-produced plasmas. Time- and space-dependent population inversion densities calculated with a collisional-radiative model used as the postprocessor of a hydrodynamic code are reported. Amplification diagnostic accuracy in plasma experiments is discussed. Time-integrated and time-resolved measurements of gain are presented, especially at 105.7 A in lithiumlike aluminum. It is shown that, in a plasma produced by a 3-nsec laser pulse, the peak of amplified radiation occurs about 7 nsec after the top of the pulse. The maximum gain–length product measured previously was 2–2.5. A short description of a future experiment designed for producing a much larger gain is presented.

New function for the RNA helicase p68/DDX5 as a modifier of MBNL1 activity on expanded CUG repeats

Myotonic Dystrophy type I (DM1) is caused by an abnormal expansion of CTG triplets in the 3′ UTR of the dystrophia myotonica protein kinase (DMPK) gene, leading to the aggregation of the mutant transcript in nuclear RNA foci. The expanded mutant transcript promotes the sequestration of the MBNL1 splicing factor, resulting in the misregulation of a subset of alternative splicing events. In this study, we identify the DEAD-box RNA helicase p68 (DDX5) in complexes assembled onto in vitro-transcribed CUG repeats. We showed that p68 colocalized with RNA foci in cells expressing the 3′UTR of the DMPK gene containing expanded CTG repeats. We found that p68 increased MBNL1 binding onto pathological repeats and the stem–loop structure regulatory element within the cardiac Troponin T (TNNT2) pre-mRNA, splicing of which is misregulated in DM1. Mutations in the helicase core of p68 prevented both the stimulatory effect of the protein on MBNL1 binding and the colocalization of p68 with CUG repeats, suggesting that remodeling of RNA secondary structure by p68 facilitates MBNL1 binding. We also found that the competence of p68 for regulating TNNT2 exon 5 inclusion depended on the integrity of MBNL1 binding sites. We propose that p68 acts as a modifier of MBNL1 activity on splicing targets and pathogenic RNA.

Amplification of soft-X-ray spontaneous emission in aluminium and magnesium plasmas

Amplification of spontaneous emission at 105.7 and 127.9 AA is reported for Li-like aluminium and magnesium in recombining laser-produced plasmas. Evidence of gain is deduced from the variation of radiation intensity according to the length of a line-shaped plasma. Detailed calculations of rate equations with a suitable plasma model, associated with numerical simulations of plasma hydrodynamics, predict population inversions between 3d and 5f levels, in accordance with these experiments.

Effective rates for li-like ions; calculated XUV gains in Al10+

This work concerns the modelling of the Li-like recombination X-ray laser. We have improved the description of the ionization dynamics and the gain production in a recombining laser-produced plasma. The contribution of the Li-like excited levels (up to n=30), which are assumed to follow a quasi-steady state evolution with respect to the ionic populations, is now incorporated in the ionization and recombination rates connecting He-like and Li-like ionic species. Simple expressions for these effective rates are derived. The number of Li-like excited levels to be included in the model, as well as the determination of the lower limit of the thermal band are examined. Predicted 3d-4f gain values (3 cm−1 at λ=154.6 Å) when simulating fibre targets cylindrically illuminated by a short laser pulse are now in good agreement with experiments. Persistent discrepancies in other cases (3d-5f inversion or slab targets) are discussed.

Soft–X-Ray Amplification in Recombining Aluminum Plasma

This paper reports time-resolved measurements of gain at 105.7 A in an aluminum plasma produced by a laser. It is shown that the amplification occurs during the plasma cooling, at the end of the laser pulse. The observed time-variation of the gain agrees with the prediction of a plasma recombination model used for calculating the level populations of the lithium-like Al10+ ions.

New spectroscopic results in the 2s2p5-2s2p43s transition array of Mg4+, Al5+ and Si6+ ions

Experimental and theoretical results on far UV transitions between levels of configurations 2s2p43s and 2s2p5 in Mg V, Al VI and Si VII spectra are reported. Theoretical analysis by means of ab initio and semi-empirical methods leads to a new identification of a number of lines, nine of which pertain to the Si VII spectrum. These are situated in the 70-90 AA range, and have not been observed previously. The energy levels of the upper configurations of Al5+ and Su6+ ions are calculated in the intermediate coupling scheme. The correlation effects expressed by configuration interactions do not greatly influence the relative energy position of the analysed lines but they do have significant consequences on their relative intensities.

A direct SCF method for low-Z atomic level structure. Application to some excited levels of the Kr isoelectronic series

The spin-orbit term of the Pauli approximation has been included directly in a SCF method of the numerical multiconfigurational Hartree-Fock type. The details of the procedure are presented, and the structure of the modified Hartree-Fock equations is given and discussed. The application to the study of some resonance lines of the Kr-like ions up to Mo VII is used as a test of the method which is compared with more sophisticated calculations and with known experimental results.

Lasing near 200 Å with neon-like zinc and lithium-like sulfur

In this paper we present the recent results obtained on X-ray lasers with the new facility of LULI (Palaiseau, France). As regards the study of collisional-excitation pumping we show that the J=0–1 line of neon-like zinc, at 212 Å, exhibits a surprisingly large gain coefficient of 4.9 cm−1. A detailed comparison of the time-dependent intensities of the J=0–1 and the J=2−1 line at 267 Å leads to the conclusion that these two lines are not emitted in the same region of the plasma. On the other hand we performed a theoretical and experimental study of 5g-4f lasing line of lithium-like sulphur at 206.5 Å. For the first time, in the case of plasma recombination pumping, we did not observe any reduction of the gain coefficient when the plasma length is raised from 1 cm to 2 cm. From numerical simulation this is likely due to radiation trapping and similar processes having much less influence on 5g-4f population inversion than on previously observed lithium-like lasing transitions. These results show that large gain-length values should be obtained with ≈0.5 kJ driving laser pulses.

Experimental study of neonlike zinc J=0−1 soft X‐ray lasing at 21.2 nm

Since our first observation of intense J=0−1 lasing in neonlike zinc in early 1993, we have investigated various issues of this X‐ray laser. The results of these experiments, performed at Laboratoire pour l’Utilisation des Lasers Intenses (LULI), are briefly reviewed. Gain coefficients of ∼5 cm−1 at 21.2 nm are now routinely obtained with ≊350 J delivered onto a Zn slab in ∼650 ps pulses—the net irradiance is ∼1.4×1013 W cm−2. The 21.2 nm emission appears in a ≊100 ps burst and precedes the lasing on the J=2−1 transitions at 26.2 and 26.7 nm which have a duration ≊350 ps. We have directly measured the gain regions of the J=0−1 line at 21.2 nm and the J=2−1 line at 26.7 nm and confirmed the initial claim that they are located at different distances from the target. We have also measured the refraction angles and divergences of the XRL beams of these two transitions. The beam at 21.2 nm was observed to be remarkably narrow with a divergence of ∼2.6 mrad, peaking ∼7 mrad off‐axis. This contrasts to the beam ...