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- Chen, Peng, 1973-
(författare)
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Function of wobble nucleoside modifications in tRNAs of Salmonella enterica Serovar Typhimurium
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Transfer RNA from all organisms has modified nucleosides and position 34 (the wobble position) is one of the most extensively modified positions. Some wobble nucleoside modifications restrict codon choice (e.g. 5-methylaminomethyl-2-thiouridine, mnm5s2U) while some extend the decoding capacity (e.g. uridine-5-oxyacetic acid, cmo5U). In this thesis the influence of wobble nucleoside modification on cell physiology and translation efficiency and accuracy is described.A mutant proL tRNA (proL207) was isolated that had an unmodified adenosine in the wobble position. Surprisingly, the proL207 mutant grows normally and is efficiently selected at the non-complementary CCC codon. The explanation of how an A34 containing tRNA can read CCC codon could be that a protonated A can form a base pair with C.cmo5U (uridine-5-oxyacetic acid) is present in the wobble position of five tRNA species in S.enterica. Two genes (cmoA and cmoB) have been identified that are involved in the synthetic pathway of cmo5U. Mutants were constructed in alanine, valine, proline, and threonine codon boxes which left only a cmo5U containing tRNA present in the cell. The influence of cmo5U on growth or on A site selection rates of the ternary complex was found to be tRNA dependent.During the study of the frameshift suppressor sufY of the hisC3737 frameshift mutation, a dominant mutation was found in YbbB protein, a selenouridine synthetase. The frameshifting occurs at CCC-CAA codon contexts and is specific for CAA codons, which are read by tRNAGlncmnm5s2UUG . The sufY204 mutation is a dominant mutation resulting in a change from Gly67 to Glu67 in the YbbB protein, and mediates the synthesis of several novel modified nucleosides/nucleotides (UKs) with unknown structure. The synthesis of these UKs is connected to the synthesis of cmnm5s2U34. The presence of UK on tRNAGlnU*UG reduced aminoacylation and therefore might account for the slow entry at CAA codons which could result in +1 frameshifting by P site tRNA. The selenourdine synthetase activity is not required for the synthesis of UKs. We hypothesize that an intrinsic activity that is low in the wild type protein has been elevated by the single amino acid substitution and results in the synthesis of UKs.
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| 2. |
- Chen, Zhe
(författare)
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Surface Integrity and Fatigue Performance of Nickel-based Superalloys
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Due to global warming, the demand for more efficient gas turbines has increased. A way to achieve this, is by increasing the operating temperature of the gas turbine. Therefore, nickel-based superalloys have been developed to withstand these extreme temperatures and loads, especially in the hot sections. Today, the way of operating land-based gas turbines is changing. Instead of running for long periods of time, the operation is becoming more flexible, with ever-increasing cyclic loads and number of start and stop cycles. To handle the increased stress and cycles, component resistance to fatigue failures need to be improved.Surface integrity is critical to fatigue performance, since fatigue cracks normally initiate at a surface. Machining changes the surface integrity which can result in worse fatigue resistance. The work presented in this Ph.D. thesis was conducted in collaboration with Siemens Industrial Turbomachinery AB in Finspång, Sweden. Surface integrity changes which are induced during machining and their effects on fatigue performance have been studied on the alloy Inconel 718. Inconel 718 is a widely-used nickel-based superalloy for high temperature applications in modern gas turbines.Broaching, milling, and wire electrical discharge machining, related to component manufacturing in turbo machinery industries, were included in this study. Machining induced surface defects provide preferential sites for fatigue crack initiation which influence the fatigue performance of the alloy. If compressive residual stresses are induced during machining, they benefit the fatigue life by retarding fatigue crack initiation away from surface regions. Shot peening was performed on machined Inconel 718, by which high compressive residual stresses are deliberately induced. It results in enhanced fatigue performance.The high temperatures in gas turbines generally deteriorate the surface integrity. Recrystallization often occurs in the highly deformed surface layer. Microstructural degradation, in the form of α-Cr precipitates, have also been frequently observed in the deformed surface and sub-surface microstructure. Oxidation at elevated temperatures degrades the surface integrity and thereby also the fatigue performance. Fatigue cracks are preferably initiated at oxidized surface carbides, if thermal exposure has been made prior to the test. It is even worse when high temperatures relax the beneficial surface compression induced by shot-peening and thereby lowering the fatigue resistance.Machinability of a newly developed nickel-based superalloy, AD 730TM, and the surface integrity induced during turning have also been studied in this thesis project. AD 730TM is a candidate for turbine disc applications with an operating temperature above 650 °C. At such high temperatures, Inconel 718 is no longer stable and its mechanical properties start to degrade.To summarize, the results from this thesis work show the importance of understanding surface integrity effects for fatigue applications, especially in harsh environments. Moreover, the knowledge gained through this work could be used for surface enhancement of turbine components which are subjected to a high risk of fatigue failure. These will contribute to more efficient and flexible power generation by gas turbines.
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| 3. |
- Ukhov, Ivan, 1986-
(författare)
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System-Level Analysis and Design under Uncertainty
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- One major problem for the designer of electronic systems is the presence of uncertainty, which is due to phenomena such as process and workload variation. Very often, uncertainty is inherent and inevitable. If ignored, it can lead to degradation of the quality of service in the best case and to severe faults or burnt silicon in the worst case. Thus, it is crucial to analyze uncertainty and to mitigate its damaging consequences by designing electronic systems in such a way that uncertainty is effectively and efficiently taken into account.We begin by considering techniques for deterministic system-level analysis and design of certain aspects of electronic systems. These techniques do not take uncertainty into account, but they serve as a solid foundation for those that do. Our attention revolves primarily around power and temperature, as they are of central importance for attaining robustness and energy efficiency. We develop a novel approach to dynamic steady-state temperature analysis of electronic systems and apply it in the context of reliability optimization.We then proceed to develop techniques that address uncertainty. The first technique is designed to quantify the variability in process parameters, which is induced by process variation, across silicon wafers based on indirect and potentially incomplete and noisy measurements. The second technique is designed to study diverse system-level characteristics with respect to the variability originating from process variation. In particular, it allows for analyzing transient temperature profiles as well as dynamic steady-state temperature profiles of electronic systems. This is illustrated by considering a problem of design-space exploration with probabilistic constraints related to reliability. The third technique that we develop is designed to efficiently tackle the case of sources of uncertainty that are less regular than process variation, such as workload variation. This technique is exemplified by analyzing the effect that workload units with uncertain processing times have on the timing-, power-, and temperature-related characteristics of the system under consideration.We also address the issue of runtime management of electronic systems that are subject to uncertainty. In this context, we perform an early investigation into the utility of advanced prediction techniques for the purpose of fine-grained long-range forecasting of resource usage in large computer systems.All the proposed techniques are assessed by extensive experimental evaluations, which demonstrate the superior performance of our approaches to analysis and design of electronic systems compared to existing techniques.
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