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Essays in Finance and Industrial Organization
No full textThis dissertation explores the interaction of structural estimation in empirical industrial organization and applications in finance. In the first chapter, I introduce a dynamic model of transaction costs to estimate how much dispersion in 401(k) fees is explained by differences in markups between employer sponsors. In the second chapter, I introduce a model of private fund manager dynamics and investor allocation to study the value of relationships in private investments. The third chapter introduces an empirical model of portfolio allocation to study how beliefs and risk preferences drive variation in investor portfolios
Gene Flow among Candidatus Endoriftia persephone Symbionts of Riftia pachyptila across Hydrothermal Vent Systems in the Guaymas Basin
No full textDeep-sea hydrothermal vents, which are commonly found along mid-ocean ridge systems and spreading centers in the deep ocean, are home to many endemic species of animals. While much of the deep sea is limited in primary production and readily available nutrients, mutualism between animal hosts and chemolithoautotrophic symbiotic bacteria is prevalent in hydrothermal vent systems, resulting in ecosystems which are brimming with life. Among the endemic species which can be found at hydrothermal vents is Riftia pachyptila, a siboglinid tubeworm species, which harbors the chemolithoautotrophic symbiotic bacterium, Candidatus Endoriftia persephone.
Hydrothermal vents represent island-like habitats that are often hundreds to thousands of kilometers apart, and it is still poorly understood how vent populations are connected across these vast geographic distances. Understanding the genetic connectivity among organisms leads to insights about population tolerance to change, including anthropogenic activities, such as extractive deep-sea mining. Although previous research has explored the level of dispersal and genetic connectivity among animal species at hydrothermal vent systems, little is known about the movement and gene flow of symbionts across these systems. Further, this is the first study of genetic connectivity of Ca. E. persephone populations across hydrothermal vent systems in the Guaymas Basin.
This study examined the level of gene flow across populations of Ca. E. persephone between distinct hydrothermal vent sites of the Guaymas Basin, Gulf of California, Mexico. Genetic diversity of Ca. E. persephone was investigated through metagenomic sequencing of the symbionts’ genomic DNA (gDNA). Symbiont populations were recovered from sixty-one R. pachyptila specimens collected from five hydrothermal vent sites across the Northern Trough and Southern Trough regions of the Guaymas Basin. Downstream population genomic analyses included examination of genetic variants and assessment of gene presence and absence. This study reports evidence of a high degree of Ca. E. persephone population connectivity across the Guaymas Basin. While further research is required to fully understand the drivers for this genetic homogeneity, these results do infer population stability and robustness
Discovery and characterization of biosynthetic enzymes for rare metallophore ligands
No full textMicroorganisms are a rich source of chemical inspiration. Bacteria encode enzymes capable of performing challenging chemical reactions using mechanistic logic distinct from, or even inaccessible to, synthetic chemists. The discovery and characterization of biosynthetic enzymes from bacteria presents a ripe opportunity for unearthing chemical fundamentals employed in the natural world for the construction of architecturally complex molecules. Enzymatic heteroatom–heteroatom bond formation has been a particularly productive area of recent study, with the discovery of numerous novel biosynthetic enzymes in the last two decades. One class of natural products that has increasingly been found to contain functional groups with heteroatom–heteroatom bonds are metallophores, or small molecules produced for the acquisition and sequestration of biologically-relevant metal ions. However, there is still a lack of biochemical information as to how many of these metal-binding functional groups are forged, including N-hydroxy- and N-nitroso-containing ligands that are emerging as prevalent metal chelators. This dissertation describes the bioinformatic identification of an unexplored family of putative N-nitrosating enzymes as well as the biochemical characterization of a novel heme-dependent guanidine N-oxygenase and two unusual flavin-dependent acyl-CoA dehydrogenases (ACADs), each involved in the biosynthesis of a CuII-binding metallophore. Through these investigations, we have revealed new enzymatic logic for the construction of rare metallophore ligands, providing greater insight into the chemical strategies employed by nature for synthesis, and these findings can be further leveraged for new enzyme and metallophore discovery.
Chapter 2 describes our bioinformatic exploration of the family of SznF-like N-nitrosating enzymes. We identified 426 unique proteins with homology to SznF, the first dedicated N-nitrosating enzyme to be characterized, encoded in bacterial genomes. Leveraging our mechanistic understanding of the metal-binding residues required by the multidomain enzyme SznF to perform both N-oxygenation and N-nitrosation, we utilized Sequence Similarity Networks and multiple sequence alignments to deduce which of the identified enzymes could perform either or both transformations. After examining the genomic context of sznF-like genes, we selected a biosynthetic gene cluster encoding the putative N-nitrosating enzyme from Streptomyces anulatus ATCC 11523 for further study.
Our efforts characterizing the product of the biosynthetic gene cluster (chm) from S. anulatus ATCC 11523 as the CuII-binding metallophore chalkophomycin are described in Chapter 3. An analysis of gene annotations for the core and accessory biosynthetic genes led to the generation of a predicted metabolite scaffold containing a hydroxyaromatic, heterocyclic, and unknown N-nitroso-containing functional group, suggestive of a metallophore. Metabolomic analysis of native chm-encoding bacteria as well as an engineered heterologous expression strain allowed us to confirm that the chm gene cluster is responsible for chalkophomycin production. Furthermore, stable isotope feeding experiments and LC–MS/MS-based structural characterization confirmed the biosynthetic precursors for chalkophomycin’s rare metal-binding ligands, N-hydroxypyrrole and a diazeniumdiolate.
The N-hydroxypyrrole functional group has been known in nature since the 1990s; however, the enzymes employed for its construction have never been biochemically characterized. In Chapter 4, we describe the first in vitro reconstitution of enzymatic N-hydroxypyrrole synthesis. In doing so, we discovered two flavin-dependent enzymes that convert carrier protein-bound L-proline to N-hydroxypyrrole, reminiscent of characterized pathways for functionalized pyrrole formation. However, these two enzymes, ChmG and ChmH, share low sequence homology to both known N-oxygenases and proline dehydrogenases, and they synthesize the N-hydroxypyrrole through an N-hydroxyprolyl intermediate. Characterization of these enzymes revealed a new biosynthetic login for functionalized pyrrole synthesis, in which derivatization of the heterocyclic core occurs prior to dehydrogenation, and a new class of N-oxygenases that act on proline and on protein-bound substrates. We also characterize the hydrolase ChmF, adenylase ChmK, and nonribosomal peptide synthetase ChmL, which are responsible for mobilization of N-hydroxypyrrole so it may be incorporated into the final natural product. Collectively, we reconstituted the entire biosynthetic pathway of N-hydroxypyrrole biosynthesis, in turn assigning function for 7 Chm enzymes.
Our efforts to biochemically characterize diazeniumdiolate biosynthesis are described in Chapter 5. SznF-like enzymes have been implicated in diazeniumdiolate biosynthesis; however, the discrete transformations and enzymes that catalyze them have not been characterized. We discovered that ChmN, a heme-dependent enzyme, performs two hydroxylations of L-arginine to afford L-Nδ,Nω-dihydroxyarginine. As the third biochemically characterized member of an emerging metalloenzyme family, the discovery of its ability to perform two hydroxylation reactions is a new activity for this enzyme class. Efforts towards reconstituting N-nitrosation by the enzyme ChmM are discussed; however, all attempts have been unsuccessful thus far. We propose involvement of the NRPS ChmO for diazeniumdiolate biosynthesis, and work to purify this enzyme for in vitro studies is described
Wintertime Cold Extremes: Mechanisms and Teleconnections with the Stratosphere
No full textWintertime cold air outbreaks are predicted to decline in frequency and intensity as a result of climate change, but in spite of a robust warming trend over the last few decades it is unclear whether this decline has been observed. Some studies have found no trend or even a slight increase in North American cold air outbreaks, which is particularly remarkable when one considers the enhanced warming signal of 2-3 degrees C already observed in the wintertime Arctic, where most air masses resulting in mid-latitude cold air outbreaks originate. But with evidence from warmer climates in the far distant past, we know that cold air outbreaks should decline with global warming. Fossils of frost-intolerant species dating back to the Eocene warm climate period and found in the interior of North America indicate that the wintertime temperature never dropped below freezing while the average temperature was only 10 degrees C warmer than it is today, implying that cold extremes warmed by 2-3 times the average. This suggests that some mechanism may be acting to maintain cold air outbreaks in the modern climate in spite of the overall warming trend. Climate models have long-standing problems matching proxy records for winter temperatures at high latitudes during the Eocene, indicating that such a mechanism may be missing or improperly represented in models. Winter weather can also be mediated by teleconnections with geographically and dynamically distinct features. The stratospheric polar vortex has been hypothesized to exert a downward influence on surface weather, and more specifically to affect the frequency of cold air outbreaks, but the time scale and nature of this influence remains elusive. My dissertation is therefore presented in two parts: chapters one and two concern the mechanisms driving cold air outbreaks in near-modern and paleo climates, while chapter three considers the nature of teleconnections between the troposphere and the stratosphere in winter.
The first chapter looks at cold air outbreaks in a pre-industrial climate. Using output from the Community Earth System Model (CESM), I identify hundreds of cold air outbreaks over the deep interior of North America. To understand how those cold air outbreaks developed, I follow the air masses backward in time to see where they came from and how they evolved as they were swept from the Arctic into the mid-latitudes. I discover a significant role for diabatic cooling of the air masses on their way to produce cold air outbreaks, where negative surface sensible heat fluxes are mixed upwards by turbulence to cool near-surface air masses. I also identify an abrupt cutoff in the surface temperature distribution at the freezing temperature, which may indicate a significant role for the latent heat released by surface water as it freezes in maintaining above-freezing temperatures in warmer climates. In the second chapter, I expand this analysis to a comparison between a pre-industrial and a much warmer (Eocene-like) climate scenario. Here I find that increases in Arctic temperatures at the origin of cold air masses are responsible for the suppression of cold air outbreaks in much warmer climates. Surprisingly, the net diabatic temperature change along these air masses is the same in the two climate scenarios, where a balance between longwave cooling and warming from boundary layer mixing is maintained as both effects become stronger in the warmer climate.
The third chapter investigates teleconnections between the troposphere and the stratosphere in both the upward and downward directions. I use Maximum Covariance Analysis, which is based on singular value decomposition, on pairs of tropospheric and stratospheric fields over 60 years of reanalysis output to identify both the time scales and the spatial patterns of covariability. I find that the greatest covariance between the troposphere and the stratosphere occurs when the surface precedes the stratosphere by up to 9 days. Unlike previous studies, which focused on the time scale itself, this analysis method also enables me to identify the surface precursor, a wave-1 pattern in sea level pressure that is followed by changes in stratospheric potential vorticity, zonal wind, and EP flux. A second sea level pressure anomaly, similar to the first but rotated about the pole by about 60 degrees, was also found to precede stratospheric EP flux variations 2-3 days later. Counter to previous studies, which have suggested an important role for the stratosphere in surface cold air outbreaks, I find little evidence for a downward influence on minimum surface temperatures
Widow Mountain
No full textAbstract
“Do you want freedom or peace?” So asks Leanore, a healer and protector of the women who seek her services. Growing up in a violent home, Leanore knows what abuse looks like in all its subtleties and iterations. The trauma of witnessing the violence of her father shaped her world view. Having sworn she will never suffer the same fate as her mother, she is horrified to find in marrying Charlie she has married a carbon copy of her father. She enlists her sister, Nadine, to help her escape. But when the plan goes awry, leaving her sister dead, and her husband Charlie, barely clinging to life, she makes a run for it. She finds herself on Widow Mountain, a world within a world, a sanctuary devoid of men and a place for women to heal.
There she meets Ora, an elderly woman, and the sage of the mountain, who teaches her a different route to freedom. It has been nearly half a century since Leanore stumbled up Widow Mountain looking for refuge. Nearly as many since she inherited her place as sage and protector from Ora. Now, it is time for Leanore to move on. She can feel it in her bones. But first, she must find the new heir to the mountain. And she must settle one final score with the man who still haunts her
What lies between China’s nationalism and growth?
No full texthttps://eastasiaforum.org/2021/07/16/what-lies-between-chinas-nationalism-and-growth/China’s competitive motivation, in areas from trade to maritime disputes, reflects a spreading national consciousness among many Chinese citizens. Western commentators often depict Chinese nationalism and patriotic pride as harmful for China — a propaganda-derived burden which sows inter-ethnic conflict, protectionism against foreign businesses and tension with neighbouring countries. These interpretations can be true to some extent for nationalism in any country but are one-sided and view Chinese nationalism and identity narrowly and without context. Chinese nationalism resembles the basic spirit of ‘national’ or ‘patriotic’ pride in other countries and extends from aspirations to win China prestige in a world of nation-states.Version of Recor
A Novel Mechanism of Killing Antibiotic-Resistant Enterococci
No full textThe enterococci are a tenacious genus of bacteria that are both commensals of nearly all land animals and hospital-associated antibiotic-resistant pathogens due to their unusual ability to tolerate antibiotics, disinfectants, desiccation, and starvation. When pathogenic strain Enterococcus faecalis V583 was grown together with commensal strain E. faecalis OG1RF, the commensal unexpectedly killed the pathogen. Previous work determined that two mobile elements in the pathogen, pheromone-responsive plasmid pTEF2 and novel integrative conjugative element NovICE, were necessary for this killing phenomenon. We created a strain of E. faecalis with pTEF2 and EF0150, the gene from NovICE necessary and sufficient for killing, fixed in place with antibiotic resistance genes, and generated pheromone-killing resistant mutants. 130 mutants were sequenced and computationally analyzed for the mutations that conferred resistance. These results identify candidates for the necessary factor on pTEF2 for the killing phenomenon and suggest possibilities as to the mechanism of killing downstream of the interaction of EF0150 and the factor from pTEF2. In addition to further illuminating how pheromone-responsive plasmid pTEF2 compares to the model enterococcal sex pheromone pCF10 and shedding light on the tradeoffs of mobile element accretion for pathogens, this work contributes to our understanding of a novel mechanism of killing enterococci, identified as a “serious threat” by the CDC, and could lead to the creation of a therapeutic based on this new target
A Diasporist Guide to Camping Here
No full textAt Camp Doikayt, the landscape is a vehicle for remembering histories of diaspora and reimagining Jewishness beyond Zionism. Summer camps proliferated in the United States after the Shoah, using the Zionist invention of the “muscular” Jew, to cultivate Jewish continuity. Camp Doikayt proposes an alternative modeled after the Jewish Labor Bund’s concept of “hereness.” In the form of a guidebook, the camp's design unfolds through a set of rules that ritualizes a land ethic of solidarity and participation. As camp wanders year to year to different abandoned Jewish sites in the Catskills, we reconfigure the materials, adapt ecological remnants, and reinterpret Jewish cultural memory
Multi-agent Motion Planning for Collaborative and Large-scale Applications
No full textIntegrating multi-robot systems into applications such as automated warehouses, environmental monitoring, and other complex domains has revolutionized various aspects of the modern world. These systems, however, bring forth significant challenges. One of the primary challenges in multi-robot systems is enabling decentralized collaboration. In scenarios where robots must operate independently without a central controller, achieving effective coordination becomes difficult due to limited global information, communication constraints, and the dynamic nature of the environment. Another significant challenge lies in the motion planning of a large number of robots in continuous environments. As the number of robots increases, so does the complexity of finding safe and efficient trajectories through dynamic, obstacle-laden spaces.
This dissertation focuses on creating practical solutions to address these challenges, which are crucial for effectively deploying multi-robot systems in real-world environments. To tackle decentralized collaboration, this work develops a consensus-based approach that enables robots to coordinate effectively despite limited global information and communication constraints. To ensure safety and efficient planning in the continuous environment, this dissertation introduces a Mixed-Integer Linear Programming (MILP) framework and techniques that improve its computational efficiency. A novel hybrid graph representation of the environments is proposed to achieve faster computation than fully continuous planning, and a safe-interval motion planning algorithm is designed. To manage a large number of agents in the continuous space, a traffic-aware two-level strategy is developed. The effectiveness of these methods is demonstrated through extensive simulations and real-world experiments, proving their potential for collaborative and large-scale applications.
The structure of the dissertation is as follows:
Chapter 1 introduces the key challenges in multi-agent motion planning and decentralized collaboration, outlining the motivation behind the research and its relevance to real-world applications.
Chapter 2 introduces a consensus-based framework that allows robots to collaborate effectively without relying on a central controller. This framework employs distributed estimation and control techniques, enabling each robot to make decisions based on local information while contributing to the overall system's goals. The chapter demonstrates the effectiveness of the proposed method through simulations and real-world experiments.
Chapter 3 presents a mixed-integer linear programming (MILP) approach to optimizing the trajectories of multiple robots navigating through environments, ensuring that the generated paths are both safe and efficient. This chapter also introduces techniques to enhance computational efficiency, making the approach feasible for real-time applications in large-scale robotic systems.
Chapter 4 introduces a hybrid graph-based environment representation that lies between discrete and continuous representations. This hybrid approach leverages the strengths of each representation, allowing for faster computation and more scalable solutions without compromising the quality of the solution.
Chapter 5 explores a traffic-aware planning strategy with a dual-level approach. High-level planning minimizes congestion by intelligently routing robots through the environment, while low-level tracking ensures precise and collision-free movement. Simulations demonstrate the effectiveness of this method in 100-agent experiments
The Psychosocial Value of Employment: Evidence from a Refugee Camp
No full textEmployment may be important to well-being for reasons beyond its role as an income source. This paper presents a causal estimate of the psychosocial value of employment in refugee camps in Bangladesh. We involve 745 individuals in a field experiment with three arms: a control arm, a weekly cash arm, and an employment arm of equal value. Employment raises psychosocial well-being substantially more than cash alone, and 66 percent of the employed are willing to forgo cash payments to continue working temporarily for free. Despite material poverty, those in our context both experience and recognize a nonmonetary, psychosocial value to employment.Version of Recor