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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, Yun
(författare)
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Smooth muscle hypertrophy and the IGF-system
- 1996
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Insulin-like growth factor-! (IGF-I) has both metabolic and mitogenic effects on smooth muscle cells (SMCs). The effects of IGF-I are modified by a group of binding proteins (IGFBPs). The present study was devoted to smooth muscle hypertrophy and the IGF-system in smooth muscle under different conditions. In urinary bladder, smooth muscle hypertrophy, initiated by partial outletobstruction, was associated with a transient increase in IGF-I mRNA, and pronounced, sustained increases oflGFBP-2 and IGFBP-4 mRNA, as well as increased protein contents of IGF-I and IGFBP-2. Regression of smooth muscle hypertrophy was associated with normalization of levels ofiGF-I, IGFBP-2 and IGFBP-4 mRNA. Expression of the IGF-I receptor did not change significantly. In portal vein, IGF-I mRNA and IGF-1 immunoreactivity were increased inhypertrophy induced by partialligation of the portal vein. Abdominal coarctation caused a rapid hypertensive response accompanied by an increased wet weight of aortic media. This was coincident with a progressive increase in aortic IGFBP-2 mRNA, about 10-fold after 14 days. The levels of IGFBP-4 mRNA in different muscle tissues and liver were decreased by diabetes and fasting, while IGFBP-2 mRNA was regulated in an organspecific 1nanner: with a sustained increase in liver and a decrease in aortic smooth muscle. Smooth muscle hypertrophy also occured in the urinary bladder of diabetic rats. DNA synthesis was increased and peaked at 2 days after induction of diabetes. DNA content per bladder wet weight was decreased by 7 days. Initially there was no changes in IGF-I mRNA, while IGFBP-2 mRNA and protein in the bladders were increased and peaked by 7 days. IGFBP-4 mRNA increased only on day 7. The changes of mRNA in bladder differed from that in liver and aorta, and suggested an early effect of stretching of the bladder due to diuresis, and later a contribution by the diabetic state. In cultured vascular SMCs, mechanical strain stimulated protein synthesis, but had little effect on DNA synthesis. However, mechanical strain potentiated the actions of IG:F'-1 and serum on both protein- and DNA synthesis, and influenced the effects of IGFBP-2. In conclusion, development of smooth muscle hypertrophy is associated with specific changes in IGF-I, IGFBP-2 and IGFBP-4, suggesting that the IGF-system may play a role in this process.
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| 3. |
- Xia, Chen
(författare)
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Development of Natural Mineral Composites for Low-Temperature Solid Oxide Fuel Cells
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Solid oxide fuel cells (SOFCs) have attracted growing attention worldwide because of their high conversion efficiency and low emissions when paired with clean fuel sources. Currently, reducing the temperature of SOFC to a low-temperature (LT) range is a mainstream trend of SOFC research. One effective way to reach this target is to explore alternative electrolytes that can maintain a desirable ionic conductivity at low temperatures. Meanwhile, it has been found that natural minerals hold great potential as functional materials for energy conversion technologies, especially ion-conducting hematite and rare-earth oxides. This thesis presents an experimental investigation of novel composite electrolytes based on two common natural minerals: hematite (LW) (α-Fe2O3) and La0.33Ce0.62Pr0.05O2-δ (LCP) for LT-SOFCs application. Initially, hematite (LW) and LCP are characterized and demonstrated as electrolytes in SOFCs. It is found the hematite ore is a mixture of α-Fe2O3, silica, and calcite, while the LCP mineral is a La/Pr co-doped CeO2. Both hematite (LW) and LCP cells exhibit encouraging performance with power densities of 150-225 and 295-401 mW cm-2 at 500-600 ℃, respectively.Following above findings, two mineral based nanocomposites – hematite-LCP and LCP/K2WO4 – are developed. Electrochemical and electrical studies reveal that the hematite-LCP gains a significantly enhanced conductivity (0.116 S cm-1 at 600 ℃) compared to individual hematite (LW) and LCP. The hematite-LCP based SOFC exhibits attractive power densities of 386-625 mW cm-2 at 450-600 ℃. Further investigation indicates that heterophasic interfacial conduction plays a crucial role in resulting in the good performance. Another composite LCP/K2WO4 is synthesized from LCP and tungstate through a wet-chemical route. The obtained composites exhibit enhanced grain boundary conduction compared to that of LCP. The composition dependence of the electrical conductivity has been studied, indicating that 90 wt% LCP/10 wt% K2WO4 is the optimum proportion with highest ionic conductivity and negligible electronic conductivity. The corresponding SOFC displays the highest power density of 500 mW cm-2 at 550 ℃. Furthermore, by incorporating a semiconductor La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) into LCP and hematite-LCP, respectively, two semiconducting-ionic composites LCP-LSCF and hematite/LCP-LSCF are designed. Crystallographic and morphological characterizations are carried out to gain insight into the material features, and the two composites are applied as the intermediate membrane layer in LT electrolyte-layer free fuel cells (EFFCs). Investigations in terms of conductivity and fuel cell performance reveal that the two composites obtain improved ionic conductivities and cell power outputs compared with those of LCP and hematite-LCP. It is also found the two composites possess mixed ionic and electronic conductivities, which are balanced in the optimal composites. Additionally, stability and Schottky junction of the best-performance EFFC are studied to verify its reliability.
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