Theoretical analysis of aliasing noises in cold atom Mach-Zehnder interferometers

We present a theoretical analysis of aliasing noises that might appear in cold atom Mach-Zehnder interferometers used for the measurement of various physical quantities. We focus more specifically on single cold atom gyroscopes. To evaluate the level of aliasing noises, we have developed a model based on the power spectral densities of the different identified noise sources as input parameters and which makes use of a servo-loop to realize a precise measurement of the rotation rate. The model allows one to take into account different modes of operation, like a continuous as well as a pulsed or even a multi-ball operation. For monokinetic atoms, we show that the intermodulation noise can be completely filtered out with a continuous mode of operation and an optimum modulation scheme for any modulation frequency but also with a pulsed operation however only for specific launching frequencies. In the case of a real continuous atomic beam having a velocity distribution, it comes out that a high attenuation can be reached which indicates clearly the potential stability improvement that can be expected from a continuous operatio

A Functional Genomics Approach to Establish the Complement of Carbohydrate Transporters in Streptococcus pneumoniae

The aerotolerant anaerobe Streptococcus pneumoniae is part of the normal nasopharyngeal microbiota of humans and one of the most important invasive pathogens. A genomic survey allowed establishing the occurrence of twenty-one phosphotransferase systems, seven carbohydrate uptake ABC transporters, one sodium∶solute symporter and a permease, underlining an exceptionally high capacity for uptake of carbohydrate substrates. Despite high genomic variability, combined phenotypic and genomic analysis of twenty sequenced strains did assign the substrate specificity only to two uptake systems. Systematic analysis of mutants for most carbohydrate transporters enabled us to assign a phenotype and substrate specificity to twenty-three transport systems. For five putative transporters for galactose, pentoses, ribonucleosides and sulphated glycans activity was inferred, but not experimentally confirmed and only one transport system remains with an unknown substrate and lack of any functional annotation. Using a metabolic approach, 80% of the thirty-two fermentable carbon substrates were assigned to the corresponding transporter. The complexity and robustness of sugar uptake is underlined by the finding that many transporters have multiple substrates, and many sugars are transported by more than one system. The present work permits to draw a functional map of the complete arsenal of carbohydrate utilisation proteins of pneumococci, allows re-annotation of genomic data and might serve as a reference for related species. These data provide tools for specific investigation of the roles of the different carbon substrates on pneumococcal physiology in the host during carriage and invasive infection

Biologia

Bacillus licheniformis a-amylase (BLA) is a highly thermostable enzyme which is widely used in biotechnological processes. Although it is produced by a non-thermophilic bacterium, it remains active for several hours at temperatures over 90degreesC under conditions of industrial starch hydrolysis. It is also far more thermostable than the alpha-amylases from B. stearothermophilus and B. amyloliquefaciens despite the strong sequence similarities between these three proteins. BLA provides therefore an interesting model for protein engineers investigating enzyme thermostability and thermostabilization. Over the last decade, we have performed an extensive mutational and structural analysis on BLA in order to elucidate the origin of its unusual thermal properties and, if possible, increase its thermostability even further. Before the three-dimensional structure was known, we had used "blind" mutagenesis and identified two critical positions where amino-acid substitutions could either increase or decrease significantly the rate of irreversible thermoinactivation. Once a detailed X- ray structure of BLA was solved, structure-based mutagenesis was used to probe the role of residues involved in salt- bridges, calcium-binding or potential deamidation processes. Our results revealed the key role of domain B and its interface with domain A in determining the overall thermostability of BLA. Most of the mutations we introduced in this region modify the stability in one way or another by influencing the network of electrostatic interactions entrapping a Ca-Na-Ca metal triad at the domain A/B interface. In the course of this mutational study we have constructed over 500 BLA variants bearing single or multiple mutations, among which many were found to be either highly detrimental or slightly beneficial to the stability. The cumulative effect of the mutations enabled us to modulate the enzyme stability over a 50degreesC temperature range without perturbing significantly the amylolytic function. Although a full understanding of the origin of BLA natural thermoresistance has not yet been reached, our study demonstrated that it is not optimized and that it can be increased or decreased artificially by several means