Autoregulation of the Escherichia coli melR promoter: repression involves four molecules of MelR

The Escherichia coli MelR protein is a transcription activator that autoregulates its own promoter by repressing transcription initiation. Optimal repression requires MelR binding to a site that overlaps the melR transcription start point and to upstream sites. In this work, we have investigated the different determinants needed for optimal repression and their spatial requirements. We show that repression requires a complex involving four DNA-bound MelR molecules, and that the global CRP regulator plays little or no role

Comparative analyses imply that the enigmatic sigma factor 54 is a central controller of the bacterial exterior

Contains fulltext : 95738.pdf (publisher's version ) (Open Access)BACKGROUND: Sigma-54 is a central regulator in many pathogenic bacteria and has been linked to a multitude of cellular processes like nitrogen assimilation and important functional traits such as motility, virulence, and biofilm formation. Until now it has remained obscure whether these phenomena and the control by Sigma-54 share an underlying theme. RESULTS: We have uncovered the commonality by performing a range of comparative genome analyses. A) The presence of Sigma-54 and its associated activators was determined for all sequenced prokaryotes. We observed a phylum-dependent distribution that is suggestive of an evolutionary relationship between Sigma-54 and lipopolysaccharide and flagellar biosynthesis. B) All Sigma-54 activators were identified and annotated. The relation with phosphotransfer-mediated signaling (TCS and PTS) and the transport and assimilation of carboxylates and nitrogen containing metabolites was substantiated. C) The function annotations, that were represented within the genomic context of all genes encoding Sigma-54, its activators and its promoters, were analyzed for intra-phylum representation and inter-phylum conservation. Promoters were localized using a straightforward scoring strategy that was formulated to identify similar motifs. We found clear highly-represented and conserved genetic associations with genes that concern the transport and biosynthesis of the metabolic intermediates of exopolysaccharides, flagella, lipids, lipopolysaccharides, lipoproteins and peptidoglycan. CONCLUSION: Our analyses directly implicate Sigma-54 as a central player in the control over the processes that involve the physical interaction of an organism with its environment like in the colonization of a host (virulence) or the formation of biofilm

New avenues for failure analysis

As evidenced by the presentations at ICEFA-3, failure in electronics equipment has received little attention from the Failure Analysis Community. Electronics is the World’s largest industrial sector, fiercely competitive and facing challenges from environmental demands and continuous miniaturisation. The paper outlines the basic elements in electronic equipment, typical operating conditions, common defects and the likely modes of failure in service. It demonstrates the greater resilience required by solders than most traditional alloys at their peak operating temperatures. With rapidly changing technologies, employment of new alloy systems and dimensions such that properties can no longer be described by bulk measurements, it is contended that the scope for failure and failure analysis is significant and growing

Defects in electronics and their significance for structural integrity

With Electronics likely to become a major area of interest to structural integrity practitioners, the paper aims to introduce the field from a structural integrity perspective. The Industry is presently facing two major challenges: continued miniaturisation and the implementation of lead-free technology associated with interconnections. Following a consideration of the background of both aspects, a brief outline of typical components and production processes is presented. Attention is then focussed upon possible defects that may arise during manufacture, and under the, often, complex conditions of service. Comparisons are drawn with more traditional applications of structural integrity, and the fresh challenges associated with the utilisation of a new generation of solder alloys, and with micro and near-nano dimensions, are introduced

Damage Produced in Solder Alloys during Thermal Cycling

The anisotropy of tin is associated with significant variations in its coefficient of thermal expansion and elastic modulus, with crystallographic direction. Under pure thermal cycling (with no externally applied stress or strain), substantial strains, in excess of 100%, may develop locally, and for very small structures, such as soldered interconnections comprising a few grains, structural integrity may be adversely affected. To examine this possibility, freestanding samples of tin, Sn-3.5wt.%Ag, Sn-0.5wt.%Cu, and Sn-3.8wt.%Ag-0.7wt.%Cu, have been subjected to thermal cycling. Temperature cycles from 30 degrees C to 125 degrees C or from -40 degrees C to 55 degrees C initially caused surface cracking, with openings up to several tens of microns after 3,000 cycles. Subsequently, the surface cracks grew into the interior of the specimens, with the maximum penetration ranging from a few microns after 100 cycles to more than 200 pm after 3,000 cycles. The cracks initiated from damage accumulated along grain boundaries. For the same temperature range, less damage resulted after the lower maximum (or mean) temperature cycle, and there appears to be a thermally activated component of cracking. The microstructure produced by rapid cooling (water quenching) was slightly more resistant than that formed by air, or furnace, cooling. Apart from microstructural coarsening, no damage accrues from isothermal exposure alone

Effects of strain rate and temperature on the stress–strain response of solder alloys

To ensure reliable design of soldered interconnections as electronic devices become smaller, requires greater knowledge and understanding of the relevant mechanical behavior of solder alloys than are presently available. The present paper reports the findings of an investigation into the monotonic tensile properties of bulk samples of three solder alloys; a lead–tin eutectic and two lead-free solders (tin–3.5 copper and a tin–3.5 silver alloy). Temperatures between–10 and 75°C and strain rates between 10–1 and 10–3 s–1 have been studied. Both temperature and strain rate may have a substantial effect on strength, producing changes well in excess of 100%. Strength is reduced by lowering strain rate and increasing temperature, and Sn–37 Pb is usually most sensitive to the latter. Expressions for strain and strain rate hardening have been developed. The Sn–0.5 Cu alloy is usually the weakest and most ductile. Sn–37 Pb is strongest at room temperature but with increasing temperature and lower strain rates it becomes inferior to Sn–3.5 Ag. Ductility changes with temperature and strain rate for all three alloys are generally small with inconsistent trends. The role of such data in stress analysis and modeling is considered and the paramount importance of employing data for conditions appropriate to service, is emphasized

The mechanical properties of lead-containing and lead-free solders—meeting the environmental challenge

Reliability is a principal objective in electronics equipment. This imposes a significant challenge, particularly in the face of continued miniaturization, which results in more severe conditions for the interconnection. At present, no sound methodologies exist for reliable life prediction of solder joints and, for lead-free alloys, this problem is accentuated by a shortage of data on mechanical behaviour appropriate to service. To meet the demands of forthcoming environmental legislation, a new generation of lead-free solder alloys is being developed. The paper presents a comparison of the mechanical behaviour of the conventional Sn-37Pb solder with that of Sn-3.5Ag and Sn-0.5Cu alloys (where the compositions are in weight per cent) which are potential replacements. It is demonstrated that the mechanical properties of the lead-free alloys may be better or worse than those of the eutectic Sn-Pb alloys. The ranking of the alloys may change according to the test conditions. Strengths fall to quite low levels with decreasing strain rate and increasing temperature. The Sn-3.5Ag solder has substantially superior creep resistance although, for creep lives less than 1000 h at 75°C, prior ageing impairs this performance, as is the case for Sn-37Pb. During high-strain fatigue, the lead-free alloys are marginally inferior, and the presence of a dwell in the strain cycle generally causes a significant reduction in life. The implications of these findings in relation to the implementation of lead-free technology are considered