Bacterial enhancer binding proteins - AAA+ proteins in transcription activation

Bacterial enhancer-binding proteins (bEBPs) are specialised transcriptional activators. bEBPs are hexameric AAA+ ATPases and use ATPase activities to remodel RNA polymerase (RNAP) complexes that contain the major variant sigma factor, σ54 to convert the initial closed complex to the transcription competent open complex. Earlier crystal structures of AAA+ domains alone have led to proposals of how nucleotide-bound states are sensed and propagated to substrate interactions. Recently, the structure of the AAA+ domain of a bEBP bound to RNAP-σ54-promoter DNA was revealed. Together with structures of the closed complex, an intermediate state where DNA is partially loaded into the RNAP cleft and the open promoter complex, a mechanistic understanding of how bEBPs use ATP to activate transcription can now be proposed. This review summarises current structural models and the emerging understanding of how this special class of AAA+ proteins utilises ATPase activities to allow σ54-dependent transcription initiation

The effect of high intensity laser propagation instabilities on channel formation in underdense plasmas

International audienceExperiments have been performed using high power laser pulses (up to 50 TW) focused into underdense helium plasmas (ne ⩽ 5×10^19 cm^−3). Using shadowgraphy, it is observed that the laser pulse can produce irregular density channels, which exhibit features such as long wavelength hosing and “sausage-like” self-focusing instabilities. This phenomenon is a high intensity effect and the characteristic period of oscillation of these instabilities is typically found to correspond to the time required for ions to move radially out of the region of highest intensity

Proposed beatwave experiment at RAL with the Vulcan CPA laser

The capacity region of the two-user Gaussian Interference Channel (IC) is studied. Three classes of channels are considered: weak, one-sided, and mixed Gaussian IC. For the weak Gaussian IC, a new outer bound on the capacity region is obtained that outperforms previously known outer bounds. The sum capacity for a certain range of channel parameters is derived. For this range, it is proved that using Gaussian codebooks and treating interference as noise is optimal. It is shown that when Gaussian codebooks are used, the full Han-Kobayashi achievable rate region can be obtained by using the naive Han-Kobayashi achievable scheme over three frequency bands (equivalently, three subspaces). For the one-sided Gaussian IC, an alternative proof for the Sato’s outer bound is presented. We derive the full Han-Kobayashi achievable rate region when Gaussian codebooks are utilized. For the mixed Gaussian IC, a new outer bound is obtained that outperforms previously known outer bounds. For this case, the sum capacity for the entire range of channel parameters is derived. It is proved that the full Han-Kobayashi achievable rate region using Gaussian codebooks is equivalent to that of the one-sided Gaussian IC for a particular range of channel parameters

A study of picosecond laser-solid interactions up to 10(19) W cm(-2)

The interaction of a 1053 nm picosecond laser pulse with a solid target has been studied for focused intensities of up to 1019 W cm-2. The maximum ion energy cutoff Emax (which is related to the hot electron temperature) is in the range 1.0-12.0 MeV and is shown to scale as Emax≈I1/3. The hot electron temperatures were in the range 70-400 keV for intensities up to 5 × 1018 W cm-2 with an indication of a high absorption of laser energy. Measurements of x-ray/y-ray bremsstrahlung emission suggest the existence of at least two electron temperatures. Collimation of the plasma flow has been observed by optical probing techniques. ©7997 American Institute of Physics