Browsing by Author "Qin, Nan"

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    Analysis of the Regulons Controlled by Transcriptional Regulators LuxR and LitR in Vibrio fischeri
    Qin, Nan (Virginia Tech, 2008-07-22)
    Quorum sensing is a bacterial signaling system that controls gene expression in a population density-dependent manner. In Gram-negative proteobacteria, the cell density control of luminescence was first observed in the symbiotic marine bacterium Vibrio fischeri and this system is one of the best studied quorum sensing systems. Two-dimensional sodium dodecyl sulfate-polyacrylamide (2D-SDS) gel electrophoresis analysis previously identified several non-Lux proteins in V. fischeri MJ-100 whose expression was dependent on LuxR and 3-oxo-hexanoyl-L-homoserine lactone (3-oxo-C6-HSL). A lacZ reporter was used to show that the promoters for qsrP, acfA, and ribB were directly activated via LuxR-3-oxo-C6-HSL in recombinant Escherichia coli. The sites of transcription initiation were established via primer extension analysis. Based on the position of the lux box-binding site near position â 40, all three promoters appear to have a class II-type promoter structure. Real-time reverse transcription-PCR was used to study the temporal expression of qsrP, acfA, and ribB during the exponential and stationary phases of growth, and electrophoretic mobility shift assays were used to compare the binding affinities of LuxR to the promoters under investigation. In order to fully characterize the LuxR regulon in V. fischeri ES114, microarray analysis was performed in the Greenberg lab (University of Washington) and 18 LuxR-3-oxo-C6-HSL regulated promoters were found including 2 genes (qsrP and acfA) identified previously in MJ-100 in addition to the well-studied lux operon. In collaboration with them, full-length purified LuxR protein was used to show direct interaction between the LuxR protein and 7 genes/operons newly identified out of 13 genes/operons examined. The binding affinity between LuxR proteins and those genes was also measured. Based on the sequence of the lux boxes of the known genes regulated by LuxR and LitR, a position specific weight matrix (PSWM) was created and used to search through the intergenic regions of the V. fischeri ES114 genome. Some potential LuxR and LitR-regulated genes with high score were tested experimently to confirm direct activation. For the LuxR regulon, these possible LuxR-regulated promoters were cloned into a lacZ reporter and tested for their LuxR dependence. Beyond the genes found in microarray, the promoter of the intergenic region VFA0658-0659 was found to be activated by LuxR and 3-oxo-C6-HSL. For the LitR regulon, two LitR-regulated genes found in the microarray were also identified using PSWM and confirmed by real-time PCR to be dependent on LitR for expression. EMSA experiments showed that LitR can specifically bind to the litR boxes of LitR-regulated genes, litR and VF0170 which confirmed that the regulation is direct.
  • Indexing and Stock Price Efficiency
    Qin, Nan; Singal, Vijay (Wiley, 2015-07-28)
    Indexing has experienced substantial growth over the last two decades because it is an effective way of holding a diversified portfolio while minimizing trading costs and taxes. In this article, we focus on one negative externality of indexing: the effect on the efficiency of stock prices. Based on a sample of large and liquid US stocks, we find that greater indexing leads to less efficient stock prices, as indicated by stronger post‐earnings‐announcement drift and greater deviations of stock prices from the random walk. We conjecture that reduced incentives for information acquisition and arbitrage induced by indexing and passive trading are probably the main causes for degradation in price efficiency.
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    Three essays on mispricing and market efficiency
    Qin, Nan (Virginia Tech, 2014-07-23)
    This dissertation consists of three essays. The first essay studies the impact of indexing on stock price efficiency. Indexing has experienced substantial growth over the last two decades because it is an effective way of holding a diversified portfolio while minimizing trading costs and taxes. In this paper, we focus on one negative externality of indexing: the effect on efficiency of stock prices. Based on a sample of large and liquid U.S. stocks, we find that greater indexing leads to less efficient stock prices, as indicated by stronger post-earnings-announcement drift, greater deviations of stock prices from the random walk and greater return predictability from lagged order imbalances. We conjecture that reduced incentives for information acquisition and arbitrage induced by indexing are probably the main cause of the degradation in price efficiency, but we find no evidence supporting a direct impact from passive trading or any effect through liquidity. The second essay investigates the effect of price inefficiency on idiosyncratic risk and stock returns. I finds that price inefficiency in individual stocks contributes to expected idiosyncratic volatility. If idiosyncratic risk is priced, greater price inefficiency could be associated with higher expected returns. Consistent with this hypothesis, this paper then finds a positive relation between price inefficiency and future stock returns. This return premium of price inefficiency is not explained by traditional risk factors, illiquidity, or transactions costs. It is also evidently different from the return bias related to Jensen's inequality. This paper thus provides new insights about the determinants of expected stock returns, and new supporting evidence that idiosyncratic risk is priced. The third essay examines whether the upward return bias generated by Jensen's inequality could lead to better performance of equally-weighted (EW) indexes than value-weighted (VW) index when stock prices are not fully efficient. We find that, for a wide range of U.S. stock indexes, EW indexes deliver better four-factor adjusted returns than VW ones do even after deducting transaction costs. Consistent with our hypothesis that the outperformance of EW indexes comes from mispricing, we find that this outperformance concentrates in stocks with greater mispricing, as measured by deviation of stock prices from random walk. Findings in this essay not only imply a potentially winning investment strategy, but also provide new insight into a long-term debate on causes of the outperformance of the EW indexes.