Regulation of heme biosynthesis targets the key enzyme HemA by a mechanism of protein stabilization in Salmonella typhimurium

Heme serves as a cofactor of cytochromes and catalases. It is essential for energy generation and in defense against toxic hydrogen peroxide in nearly all cells including Salmonella typhimurium and Escherichia coli. Indirect evidence has suggested that heme synthesis is a regulated process. Little is known about how heme synthesis is regulated in enteric bacteria even though the heme synthetic pathway is genetically well-defined. This research represents the first report that heme synthetic regulation affects the first committed heme pathway enzyme, glutamyl-tRNA reductase (HemA), by an unusual mechanism.;HemA, encoded by hemA gene, catalyzes the rate-limiting step of heme biosynthesis. This project demonstrated that when these bacteria are starved for heme, HemA enzyme activity and protein abundance increase 10--25 fold, while gene expression is not affected much (less than 2-fold induction). These results provide the first direct evidence that heme synthetic regulation targets HemA and suggest that the HemA regulation occurs at the post-transcriptional level.;The results of this project revealed a unique mechanism of HemA regulation by a conditional stability of the HemA protein. The half-life of HemA is about 20 min in unrestricted cells, but increases to \u3e300 min in heme-limited cells. The ATP-dependent proteases responsible for HemA turnover were discovered by testing E. coli mutants. HemA turnover is completely blocked in a lon clpP double mutant, but not in either single mutant, indicating that both Lon and ClpP are involved in HemA proteolysis. ClpA, but not ClpX was further determined to have a role in HemA degradation as the chaperone of ClpP.;The amino acids of HemA that signal degradation were determined in this project. A hybrid HemA-lacZ protein containing the first 18 amino acids of the HemA N-terminal region, is also stabilized in a lon clpP mutant. Insertion of two lysines after the second N-terminal amino acid of HemA completely stabilizes this protein while not impairing enzyme function. This finding confirms the hypothesis that HemA degradation tag lies in the N-terminus. Several models are discussed in this dissertation for the signals and regulatory components of the HemA regulation pathway

Three Essays in Finance

This dissertation is composed of three essays on corporate credit markets and corpo- rate finance. The first essay examines the impact of unconventional monetary policies (UMPs), including Quantitative Easing (QE), Operation Twist (OT), and Forward Guidance (FG), on corporate credit markets. These policies were expected to reduce credit spreads by decreasing credit risk premium and/or liquidity premium, and to further lengthen borrowing maturity. During the crisis, Quantitative Easing (QE) 1 reduced these risk premia as expected. However, after the crisis, QE 2 and Oper- ation Twist announcements increased fears of a weaker economy and, consequently, widened credit spreads. In contrast, Forward Guidance reduced credit risk premia without increasing fear premia. I also find that QE had a minimal effect on corporate bond maturities, which most likely reflected the considerable increase of new Treasury issuance and the declining fraction of preferred-habitat investors. The largest impact on corporate bond maturities came from UMPs that significantly flattened the yield curve. The second essay (published in the Journal of Fixed Income) studies the impact of margin requirements on the Credit Default Swap (CDS) basis. The CDS basis was significantly negative during the 2007-2009 financial crisis, which was considered an anomaly. Using single-name CDS data, we find that the CDS basis decreases as the funding costs, credit risk premium, and market illiquidity increase. Further, cross- sectional results show that the sensitivities of the CDS basis to funding costs, credit risk premium, and market illiquidity are priced, even after controlling for the indi- vidual bond liquidity and other firm characteristics. The results are consistent with the margin-based asset pricing theories that the difference in margin requirements on two otherwise identical securities gives rise to bases. The third essay (co-authored with Yongqiang Chu) examines the relationship be- tween a firm’s leverage and that of its customers. The bargaining theory of capital structure predicts that, when a customer increases its bargaining power by increas- ing its leverage, the supplier will raise its leverage as well in order to maintain its bargaining power. However, the relation-specific investment theory of capital struc- ture suggests an opposite relationship. An increased leverage ratio reduces the value of such investments, and, therefore, the supplier may not compete on the leverage ratio. We find that, in general, a firm’s leverage is positively associated with its customer’s leverage, and we find empirical evidence supporting both theories. The result is robust to a battery of specifications and instrumental variables

Main Problems in College Students’ Oral English Learning and the Corresponding Teaching Strategies

For a long time, college English learners attach more attention to the input of language knowledge while ignoring the output aspect in language practice, leading to the dissatisfactory phenomena—“dumb English”. This paper aims at the main problems in college students’ oral English learning and points out several prominent problems such as the internalized factors in language knowledge; meanwhile, it presents some effective strategies in English teaching such as the combination of “modeling and imitating” strategy, presenting strategy, situation creating strategy with detailed examples