Diverse pathways for salicin utilization in Shigella sonnei and Escherichia coli carrying an impaired bgl operon

Utilization of the aryl-beta-glucosides salicin or arbutin in most wild-type strains of E. coli is achieved by a single-step mutational activation of the bgl operon. Shigella sonnei, a branch of the diverse E. coli strain tree, requires two sequential mutational steps for achieving salicin utilization as the bglB gene, encoding the phospho-beta-glucosidase B, harbors an inactivating insertion. We show that in a natural isolate of S. sonnei, transcriptional activation of the gene SSO1595, encoding a phospho-beta-glucosidase, enables salicin utilization with the permease function being provided by the activated bgl operon. SSO1595 is absent in most commensal strains of E. coli, but is present in extra-intestinal pathogens as bgcA, a component of the bgc operon that enables beta-glucoside utilization at low temperature. Salicin utilization in an E. coli bglB laboratory strain also requires a two-step activation process leading to expression of BglF, the PTS-associated permease encoded by the bgl operon and AscB, the phospho-beta-glucosidase B encoded by the silent asc operon. BglF function is needed since AscF is unable to transport beta-glucosides as it lacks the IIA domain involved in phopho-relay. Activation of the asc operon in the Sal(+) mutant is by a promoter-up mutation and the activated operon is subject to induction. The pathway to achieve salicin utilization is therefore diverse in these two evolutionarily related organisms; however, both show cooperation between two silent genetic systems to achieve a new metabolic capability under selection.

Effect of favorable pressure gradient on transitional spot formation rate

The calculation of the transitional boundary layer requires estimates of the extent of the transition zone, which in turn depends on the rate at which turbulent spots are formed. This rate has been found to scale with local boundary layer thickness and viscosity, and the resulting nondimensional group (called crumble) is a function of the pressure gradient, among other parameters. Available experimental data are analyzed to show that the crumble increases slowly with increasing favorable pressure gradients, being about four times as large as in constant-pressure flow when the Thwaites pressure gradient parameter at the effective origin of the resulting turbulent boundary layer is 0.1 and when transition is driven by free-stream turbulence.

On the models for deuterium long-range isotope effects in13C NMR spectroscopy

The long-range deuterium isotope effects on13C nuclear shielding are physically not yet completely understood. Two existing models for explaining these effects, vibrational and substituent, are compared here. The vibrational model is based on the Born-Oppenheimer approximation, but it can explain only one-bond deuterium effects. To the contrary, the substituent model may explain many long-range isotope effects, but it is controversial due to the assumption of some distinct electronic properties of isotopes. We explain how long-range deuterium isotope effects may be rationalized by the subtle electronic changes induced by isotope substitution, which does not violate the Born-Oppenheimer approximation.

Orientation dependence of the indentation impression morphology in a Mg alloy

The formation of anomalous indentations, with two opposite faces describing a pin-cushion effect and the other two faces normal, in long elongated grains of an extruded Mg-2Al-1Zn alloy is reported. Subsurface microstructural observations combined with Schmid factor calculations suggest that extension twinning accompanied by basal slip are the reasons for these. Johnson's expanding cavity model is invoked for further substantiation. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Detrimental Effects of Hypoxia-Specific Expression of Uracil DNA Glycosylase (Ung) in Mycobacterium smegmatis

Mycobacterium tuberculosis is known to reside latently in a significant fraction of the human population. Although the bacterium possesses an aerobic mode of metabolism, it adapts to persistence under hypoxic conditions such as those encountered in granulomas. While in mammalian systems hypoxia is a recognized DNA-damaging stress, aspects of DNA repair in mycobacteria under such conditions have not been studied. We subjected Mycobacterium smegmatis, a model organism, to the Wayne's protocol of hypoxia. Analysis of the mRNA of a key DNA repair enzyme, uracil DNA glycosylase (Ung), by real-time reverse transcriptase PCR (RT-PCR) revealed its downregulation during hypoxia. However, within an hour of recovery of the culture under normal oxygen levels, the Ung mRNA was restored. Analysis of Ung by immunoblotting and enzyme assays supported the RNA analysis results. To understand its physiological significance, we misexpressed Ung in M. smegmatis by using a hypoxia-responsive promoter of narK2 from M. tuberculosis. Although the misexpression of Ung during hypoxia decreased C-to-T mutations, it compromised bacterial survival upon recovery at normal oxygen levels. RT-PCR analysis of other base excision repair gene transcripts (UdgB and Fpg) suggested that these DNA repair functions also share with Ung the phenomenon of downregulation during hypoxia and recovery with return to normal oxygen conditions. We discuss the potential utility of this phenomenon in developing attenuated strains of mycobacteria.

The chemistry of vetivalene-type vetivalene-type naturally-occurring sesquiterpenoids

Vetivalene, 1,4-dimetiiyl-6-isopropylnaphl1tlmlenc(1 ) represents a new sesquiterpene skelcton which 15 presumed to originate from eudesmane by a shift of the angular nicthyl group. Novel sesquiterpenm related to vetivalene have been isolated from plant sources ill ~s c e o ig ears. A suriey dealing xith the chemistry (structure, synthesis and configuration) of members of this interesting new class OF yesquiterpenes, coniprising occidol, risbitinol and the various ernmotinn is presented

The chemistry of vetivalene-type naturally occurring sesqufterpenoids

Vetivalene, 1,4-dimetiiyl-6-isopropylnaphl1tlmlenc(1 ) represents a new sesquiterpene skelcton which 15 presumed to originate from eudesmane by a shift of the angular nicthyl group. Novel sesquiterpenm related to vetivalene have been isolated from plant sources ill ~s c e o ig ears. A suriey dealing xith the chemistry (structure, synthesis and configuration) of members of this interesting new class OF yesquiterpenes, coniprising occidol, risbitinol and the various ernmotinn is presented.

Random-Restart Reactive Tabu Search Algorithm for Detection in Large-MIMO Systems

We present a low-complexity algorithm based on reactive tabu search (RTS) for near maximum likelihood (ML) detection in large-MIMO systems. The conventional RTS algorithm achieves near-ML performance for 4-QAM in large-MIMO systems. But its performance for higher-order QAM is far from ML performance. Here, we propose a random-restart RTS (R3TS) algorithm which achieves significantly better bit error rate (BER) performance compared to that of the conventional RTS algorithm in higher-order QAM. The key idea is to run multiple tabu searches, each search starting with a random initial vector and choosing the best among the resulting solution vectors. A criterion to limit the number of searches is also proposed. Computer simulations show that the R3TS algorithm achieves almost the ML performance in 16 x 16 V-BLAST MIMO system with 16-QAM and 64-QAM at significantly less complexities than the sphere decoder. Also, in a 32 x 32 V-BLAST MIMO system, the R3TS performs close to ML lower bound within 1.6 dB for 16-QAM (128 bps/Hz), and within 2.4 dB for 64-QAM (192 bps/Hz) at 10(-3) BER.

Minimum weight design of fiber-reinforced laminated composite structures with maximum stress and Tsai-Wu failure criteria

In this article, a minimum weight design of carbon/epoxy laminates is carried out using genetic algorithms. New failure envelopes have been developed by the combination of two commonly used phenomenological failure criteria, namely Maximum Stress (MS) and Tsai-Wu (TW) are used to obtain the minimum weight of the laminate. These failure envelopes are the most conservative failure envelope (MCFE) and the least conservative failure envelope (LCFE). Uniaxial and biaxial loading conditions are considered for the study and the differences in the optimal weight of the laminate are compared for the MCFE and LCFE. The MCFE can be used for design of critical load-carrying composites, while the LCFE could be used for the design of composite structures where weight reduction is much more important than safety such as unmanned air vehicles.

High pressure studies on AgI-Ag2O-MoO3 glasses

he effect of pressure on the conductivity of AgI-Ag2 O-MoO3 glasses has been reexamined. A conductivity maximum is observed around 0.7 GPa. No variation of the sample temperature is noted under pressure. The results are found to agree well with the cluster-tissue model.

Layered Tabu Search Algorithm for Large-MIMO Detection and a Lower Bound on ML Performance

In this paper, we are concerned with low-complexity detection in large multiple-input multiple-output (MIMO) systems with tens of transmit/receive antennas. Our new contributions in this paper are two-fold. First, we propose a low-complexity algorithm for large-MIMO detection based on a layered low-complexity local neighborhood search. Second, we obtain a lower bound on the maximum-likelihood (ML) bit error performance using the local neighborhood search. The advantages of the proposed ML lower bound are i) it is easily obtained for MIMO systems with large number of antennas because of the inherent low complexity of the search algorithm, ii) it is tight at moderate-to-high SNRs, and iii) it can be tightened at low SNRs by increasing the number of symbols in the neighborhood definition. Interestingly, the proposed detection algorithm based on the layered local search achieves bit error performances which are quite close to this lower bound for large number of antennas and higher-order QAM. For e. g., in a 32 x 32 V-BLAST MIMO system, the proposed detection algorithm performs close to within 1.7 dB of the proposed ML lower bound at 10(-3) BER for 16-QAM (128 bps/Hz), and close to within 4.5 dB of the bound for 64-QAM (192 bps/Hz).

Modes of binding of α(1–2) linked manno-oligosaccharides to concanavalin A

Three-dimensional structures of the complexes of concanavalin A (ConA) with alpha(1-2) linked mannobiose, triose and tetraose have been generated with the X-ray crystal structure data on native ConA using the CCEM (contact criteria and energy minimization) method. All the constituting mannose residues of the oligosaccharide can reach the primary binding site of ConA (where methyl-alpha-D-mannopyranose binds). However, in all the energetically favoured complexes, either the non-reducing end or middle mannose residues of the oligosaccharide occupy the primary binding site. The middle mannose residues have marginally higher preference over the non-reducing end residue. The sugar binding site of ConA is extended and accommodates at least three alpha(1-2) linked mannose residues. Based on the present calculations two mechanisms have been proposed for the binding of alpha(1-2) linked mannotriose and tetraose to ConA.

Mismatch repair genes of eukaryotes

Mismatches that arise during replication or genetic recombination or owing to damage to DNA by chemical agents are recognized by mismatch repair systems. The pathway has been characterized in detail in Escherichia coli. Several homologues of the genes encoding the proteins of this pathway have been identified in the yeast Saccharomyces cerevisiae and in human cells. Mutations in the human genes hMSH2, hMLH1, hPMS1 and hPMS2 have been linked to hereditary nonpolyposis colon cancer (HNPCC) and to some sporadic tumours. Mismatch repair also plays an antirecombinogenic role and is implicated in speciation.