Biology

1997 Senior Thesis Abstracts

(CLASS OF 1997: If your thesis abstract is not currently included on this page and you would like it to be, please follow this link.)

Effects of floral age in Mimulus guttatus (Scrophulariaceae): Do things really get better with age?

Leigh A. Barnes

(Advisor: Karoly)

ABSTRACT

Effects of floral age were examined for plants from the Iron Mountain population of Mimulus guttatus. Pollen viability over five days was measured by using germination on an artificial medium, Fluorochromatic reaction (FCR) and lactophenol-aniline blue tests. Stigma receptivity over seven days was measured using percent fruit set in 20 flowers for each treatment day. Corolla shedding was examined to determine whether it was a mechanism of selfing in this population. Twenty plants were subjected to a hand pollination experiment where four types of pollination were used: outcross pollen from 1 day old flowers, outcross pollen from 5 day old flowers, self pollen from 1 day old flowers, and self pollen from 5 day old flowers. Seed set and percent germination of these seeds were measured for these crosses. Progeny from each cross were measured for 5 other traits of fitness: time until flowering, flower number, proportion flowering, leaf area, and dry mass. Pollen viability was found to decline significantly over 5 days in two of the three tests performed. Stigma receptivity also declined over time. In this population it was found that corolla shedding significantly contributed to fruit set. Two of the seven fitness traits studied, leaf area and dry mass, were found to be highly depressed in individuals which were self-pollinated, while pollen age had little affect on the progeny. Two more traits, percent seed germination and total flower number, showed a trend which is indicative of what would be expected if selection on pollen viability increased fitness in progeny. Progeny parented by older self pollen were slightly more fit than plants parented by younger self pollen, though this trend was not significant. Inbreeding depression was found to be larger in the new treatment progeny than old treatment progeny.

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Fire Ecology in the Ponderosa Pine Community of the Blue Mountains in Eastern Oregon and Washington

Talicia Brown

(Advisor: Brehm)

ABSTRACT

Fire is a fundamental ecological process that shapes the structure and function of plant communities. In the ponderosa pine community of the Blue Mountains in eastern Oregon and Washington, fire-scarred tree rings indicate that low intensity burns occurred in this area approximately every ten years. This may be an underestimation because some fires may not be hot enough to cause tree-scarring. The ponderosa pine association is maintained by these recurring fires. Conversely, the association's physical, chemical, and biological factors determine fire frequency and intensity. The Forest Service policy of fire suppression has resulted in a decline in the forest health of the Blue Mountains. Fire suppression has altered three main ecological processes in this ecosystem. Succession of vegetation has shifted so that the historically open, park-like, even-aged ponderosa pine-dominated stands have been replaced by dense, stagnated, multistoried stands of Douglas fir, white fir, grand fir, and incense cedar. Stands have become more susceptible to insect and disease outbreaks, resulting in a significant increase in dead and infected trees. Fire suppression has decreased nutrient cycling, biomass decay, and site productivity. A substantial increase in fuel build-up has resulted in an increase in catastrophic, uncontrollable, high intensity wildfires. Ecosystem management policies should incorporate the use of prescribed burning to restore forest health to the Blue Mountains ponderosa pine community.

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An Investigation of the Effects of Insulin on Peptide Release from the Bag Cells of Aplysia

Keith Calkins

(Advisor: Arch)

ABSTRACT

The secretion of acidic peptide (AP) and egg-laying hormone (ELH) in response to insulin was examined in the neuroendocrine bag cells of Aplysia californica. Differential secretion of these peptides can be induced by an external stimulus, indicating that the pools of vesicles which contain each of these peptides are somehow different. Insulin has been shown to induce the secretion of ELH through pathways activated by the insulin receptor tyrosine kinase. The pathways by which secretion is stimulated have not yet been determined. In this study the bag cell organs were subjected to either high potassium or insulin secretory challenges. Due to methodological errors, radiolabeled AP and ELH secretion could not be accurately measured.

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The Effects of Mating System and Environment on Maternal Effects in Lupinus nanus subsp.apricus and Lupinus bicolor (Fabaceae)

Elizabeth M. Cauble

(Advisor: Karoly)

ABSTRACT

Maternal effects may arise from genetic differences among plants in their ability to provision seeds, genetic differences expressed in maternally derived structures such as the seed coat or seed dispersal structures, cytoplasmic factors, or from effects of the maternal environment. The expression of maternal effects was examined in greenhouse-grown populations of two species of lupine, Lupinus nanus subsp. apricus (APR) and Lupinus bicolor (BIC), grown under different nutrient conditions. The nutrient treatment had a significant effect on BIC but not APR. Effects of the maternal fertilizer treatment were present in APR, but were not significant. Initial seed weight had a significant effect on germination success. There were significant differences in the way that the two species reacted to the treatments. Maternal environmental effects improved the offspring condition for BIC in the bad environment.

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The Role of the Actin Cytoskeleton in the Structural Organization of Secretory Vesicles

Sascha Ellers

(Advisor: Arch)

ABSTRACT

The neuroendocrine bag cells of Aplysia californica produce two active peptides, acidic peptide (AP) and egg-laying hormone (ELH), from a common precursor. Despite the unitary stoichiometry of production, these peptides are differentially secreted. The cytoskeleton has been implicated as part of the differentiation mechanism. To test the role of the actin cytoskeleton in vesicle organization, bag cells were treated with either the actin depolymerizing drug cytochalasin B, or the actin stabilizing drug, phalloidin. Treated and control bag cells were homogenized and subjected to low, medium and high speed centrifugations with the assumption that differentially distributed peptides would pellet in distinct fractions. Radiolabeled peptides were separated using isoelectric focusing and the relative amount of AP and ELH in each sample was determined by liquid scintillation counting. In experiments with both treated and untreated samples the second pellet was found to be preferentially enriched in ELH, while the primary and tertiary pellets contained an abundance of AP. Since treatment with actin altering drugs did not affect vesicle distribution, the actin cytoskeleton does not appear to play a role in differential organization of the vesicles. However, the observation that ELH containing vesicles pellet after a medium speed centrifugation suggests that these vesicles form an association with a high molecular weight cytoplasmic particle. In addition, the preponderance of AP in the primary pellet implies that these vesicles preferentially associate with the plasma membrane. These results are consistent with a model of secretion in which secretory granules reside in three functionally distinct pools within the terminal. This study was funded in part by a HHMI Undergraduate Research Program grant.

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The Role of the Epithelium in the Regulation of Stromal Cell Proliferation

Cindy Ellig

(Advisor: McClellan)

ABSTRACT

Stromal-epithelial interactions are thought to be crucial in the regulation of steroid hormone responses in the female reproductive tract. It is these interactions, possibly mediated by soluble growth factors, that ensure the cellular program of hormone action. I tested the effects of 17-b estradiol and epidermal growth factor on the proliferation and survival of stromal cells, grown alone or in co-culture with epithelial cells. The presence of epithelial cells on laminin-coated inserts dramatically increased the size of stromal colonies and decreased apoptosis. Stromal cells were also grown in medium conditioned by epithelial cells in order to determine if the physical presence of the epithelial cells was necessary to increase stromal proliferation and survival. I found that conditioned medium increased the percent of mitotic stromal cells drastically compared to epidermal growth factor and 17-b estradiol. My results show that there is a reciprocal interaction whereby the epithelial cells regulate stromal proliferation. Furthermore, this regulation is mediated by a soluble paracrine factor secreted by the epithelial cells that may not be EGF, the classic estrogen-induced growth factor. This study was funded in part by a HHMI Undergraduate Research Program grant.

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Influences of abiotic environmental features on the distribution and abundance of Delphinium leucophaeum (white rock larkspur) and their implications for restoration ecology

Nancy Hoegler

(Advisor: Karoly)

ABSTRACT

The extent to which the abiotic features of an ecosystem determine the composition, distribution, and abundance of organisms living there is not known. If their role is a significant one (as it appears to be in aquatic ecosystems, for instance), ecological restoration that focuses on replacing these abiotic elements and their functions in the ecosystem first should be more successful than efforts that focus on rehabilitating the biotic components of the ecosystem.

To test the theory that abiotic environmental features determine a species' distribution, four parameters of soil quality and two sets of historical data were analyzed with respect to the patchy distribution of an endangered perennial plant, Delphinium leucophaeum, at a local nature preserve. It was hoped that this would also provide clues about appropriate mitigation efforts for this species, as rare plant mitigation is an aspect of restoration ecology. Though one of the findings suggests that inappropriate soil depth has kept the more robust subpopulations from expanding, none of the abiotic parameters tested clearly showed an influence on the distribution and number of these plants. Except for the finding concerning soil depth, the null hypotheses for this project could not be rejected.

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Development of a Kluyveromyces lactis linear double-stranded plasmid DNA system for establishing double-stranded RNA viral infections in Saccharomyces cerevisiae

Angela M. Hughes

(Advisor: Russell)

ABSTRACT

Many laboratory strains of Saccharomyces cerevisiae contain a cytoplasmic population of the L-A double-stranded RNA (dsRNA) virus and possibly one of the dsRNA viruses dependent on L-A for propagation such as M, S, or X. The life cycle of all of these dsRNA viruses is directly dependent upon the two proteins encoded by L-A, Gag and Gag-Pol, which are responsible for (+)-strand encapsidation, capsid formation, and (-)- and (+)-strand synthesis. All of these processes require unique recognition sites within the RNA sequences to facilitate encapsidation, replication, and transcription. These recognition sites are called cis-acting sequences meaning they control functions within the RNA strand on which they are located. Additionally, the M dsRNA species encode unique killer toxins and associated immunities. Maintenance of both the killer phenotype and these viral populations also involves viral-host interactions which include the products of nuclear genes necessary for viral dsRNA maintenance as well as the products of genes that oppose these processes. In order to understand better the functions of the cis-acting sequences and the viral-host interactions, a system for establishing an engineered viral population is necessary. This thesis proposes that another killer species from the yeast Kluyveromyces lactis composed of two cytoplasmic double-stranded DNA (dsDNA) linear plasmids, k1 and k2, can recombine to incorporate a cDNA of the M1 (+)-strand which is flanked by k2 sequences into an S. cerevisiae cell containing k2 and L-A. Once recombination into k2 has occurred, theoretically replication of the M1 cDNA will be under the control of an Upstream Conserved Sequence (UCS) and native k2 DNA polymerase. M1 (+)-strand transcripts of wild-type sequence are expected to interact with the L-A encoded proteins and begin the viral life cycle. To test the feasibility of this k2 system first, a very sensitive reporter gene, luciferase, was also flanked by k2 sequences and transformed into S. cerevisiae containing k1 and k2. Based on previous recombination experiments, it is assumed that recombination is occurring between k2 and the k2 sequences on either side of the luciferase gene. As expected, the luciferase gene was not expressed when it was not fused to an Upstream Conserved Sequence promoter. The next step will be to insert the promoter as well as the luciferase gene and assay for luciferase expression prior to using this system to incorporate the M1 (+)-strand cDNA.

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Limb Development and Digital Reduction in Xenopus laevis

Jennifer M. Ingham

(Advisor: Black)

ABSTRACT

Does the size of a developing limb bud influence the number of digits that the limb bud will form? Does this have any evolutionary implications for species that have more or less than the normal five digits? Alberch and Gale (J. Embryol. Exp. Morph., 76: 177-197, 1983) attempted to address these questions by treating the limb buds of Xenopus laevis tadpoles with the mitotic inhibitor colchicine. The resulting limb buds were much smaller than the controls; the digital pattern, however, was relatively unchanged. Animals usually lost either the last digit to be formed or a few terminal phalanges. Despite these minimal results it has been claimed that limb bud size is crucial to the formation of the digital pattern. In light of their minimal dataset, my thesis attempted to confirm their results.

The Alberch and Gale experiment was difficult to reproduce. Only one tadpole treated with colchicine lived more than a few days. Tadpoles were able to survive treatment with nocodozole in DMSO (which allows it to be absorbed through the skin probably because their was no direct blood exposure to the drug. These tadpoles are still growing, so the outcome is still unknown. I will also discuss the supracellular, cellular and molecular aspects of limb development.

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Effects of Water Stress on the Activities of Three Anti-Oxidative Enzymes within the Root Nodules of Two Regionally Distinct Ecotypes of Lupinus nanus

Jason S. Julian

(Advisor: Dalton)

ABSTRACT

Elevated levels of activated forms of oxygen are often produced within plant tissues in response to environmental stress. These intermediate forms of oxygen have the potential to cause significant damage to plant proteins, membrane lipids, and DNA through oxidation reactions. Plants have evolved a wide array of anti-oxidative defense mechanisms to counter this problem. Of the multiple stresses which can lead to the production of such active oxygen intermediates, one of the most common forms is water stress. Of increasing interest within the scientific literature has been the investigation into the effects which such stresses have on the agriculturally significant legume family. Within this thesis, the effects of water stress on the activities of three anti-oxidative enzymes within the nodules of two regionally distinct ecotypes of the annual legume, Lupinus nanus, were observed. It was hypothesized that an increase in water stress would result in greater oxidative damage and concomitant increases in the antioxidant levels within the plants' nodules to counteract this oxidation. It was further hypothesized that there would be a noticeable difference between such responses by the two ecotypes resulting from evolutionary adaptations to their distinct environments. The results suggest that there is an increase in nodule antioxidant response levels when plants are subjected to drought stress. The results further suggest that there is a significant difference in the two ecotypes' responses to water stress in nitrogenase activity, glutathione reductase activity, and superoxide dismutase activity, but that further experimentation would have to be performed to truly comprehend their altered expression of antioxidant defense mechanisms.

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A Search for Interleukin-10 in Xenopus laevis

Ingrid Loma

(Advisor: Ruben)

ABSTRACT

Interleukins are a group of signaling molecules involved in the communication between cells in the immune system. Interleukin-10 has been implicated as an important regulator of the function of lymphoid cells. Its ability to inhibit cytokine synthesis by Th1 cells and activate B cell proliferation and differentiation renders it as a potent regulator in immune functions, such as inflammation, graft rejections and cancer. Previous studies demonstrate that Xenopus laevis rarely develop cancer. Investigating whether Xenopus have an IL-10-like protein will help us to better understand how the immune system of amphibians has the ability to reject cancer cells. The goal of this study was to determine whether IL-10 has been conserved in evolution. Enzyme linked immunoabsorbance assays, using mouse anti-IL-10, suggest that the structure of IL-10 may have changed so much during evolution, that the binding domains in the antibodies do not recognize it in supernatant fluids of activated Xenopus splenocytes. Further, mouse-IL-10 has no effect in vitro on anti-TNP antibody production by Xenopus splenocytes under the different experimental conditions used.

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Cryptic Self-Incompatibility in Two Populations of Lupinus nanus (Leguminosae)

Maria Loosen

(Advisor: Karoly)

ABSTRACT

The existence of a cryptic incompatibility system was tested for in two populations of L. nanus, formerly known as L. nanus ssp. apricus and L. nanus ssp. latifolius. These populations are referred to in this study as the inland and coastal populations, respectively. The inland population of lupine is known to have an intermediate outcrossing rate, t = 0.4-0.66 under natural conditions. It is also known to experience variable pollinator availability. The outcrossing rate for the coastal population is 0.7-1.0. Both species are self-fertile, and both populations were examined for presence of a cryptic incompatibility system. A reciprocal crossing design was used to test the competitive ability of self and outcross pollen when present in equal amounts on the stigma. Genetic markers were used to determine paternity, Lap in the coastal population and Idh and Pgi in for the inland lupine. The seed set for the coastal lupine showed a significantly higher proportion of outcrossed seeds to selfed seeds, and an outcrossing rate of 0.76. Outcross pollen donor success was found to be significantly higher than self pollen donor success, and showed a 21% reduction from outcross siring ability to self siring ability. This population was determined to have a cryptic incompatibility system. Outcrossing did not occur more frequently than expected in the inland L. nanus, which had an outcrossing rate of 0.48. Pollen donor success analysis revealed no significant difference between outcross and self pollen success. There was a high degree of mating system variation present in this population, which did not show evidence of a cryptic incompatibility system. This study was funded in part by a HHMI Undergraduate Research Program grant.

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Characterization of Protein-DNA Interactions By Fluorescence Anisotropy: A pH Dependent Binding Study of the Purine Repressor

Steven E. Mansoor

(Advisors: Glasfeld & Gerrity - BMB)

ABSTRACT

A fluorescence anisotropy laser system was designed, built, and implemented to study the pH-dependent binding of the purine repressor (PurR) to an analog (TpC) of its operator sequence (purF). Specifically, the protonation state of a histidine residue (His20) that is believed to make a significant hydrogen bond to a guanine base on the DNA sequence was investigated. Although the laser system was functional and could qualitatively determine changes in binding, it could not be used to perform the full pH-dependent study due to high levels of noise in the data.

The binding measurements at each pH were made on a Photon Technologies International (PTI) constant white light fluorimeter. Results indicated that the His20 may possess a pKa value of around 6.0. Nonspecific, low-affinity binding to random locations on the DNA was found to occur in the solutions of pH below 6.0 (pH 5.5 and pH 4.5) while specific, saturated binding to the operator sequence was found to occur at pH values above 6.0 (pH 7.5 and pH 8.5). Thus, it was concluded that His20 of PurR is deprotonated when bound specifically to its operator sequence. Tentative results also indicated that PurR does not bind at all at pH 9.5. It is hypothesized that this might be due to a change in the protonation state of an amino acid (Tyr282) responsible for maintaining the dimer structure with which PurR binds.

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Effects of Altered Phosphorylation Levels on Progesterone Receptor Localization

Alex Monreal

(Advisor: McClellan)

ABSTRACT

Progesterone (P) plays a significant role in the complex physiological events which occur in the female reproductive tract. At the molecular level, these effects are mediated by the progesterone receptor (PR). PR action is known to be regulated by protein-protein interactions, and may also depend upon subcellular compartmentalization, and phosphorylation. Evidence exists which suggests that phosphorylation may play a role in subcellular localization of numerous transcription factors including steroid receptors. This study examined the effects of global alterations of levels of phosphorylation of cellular proteins on PR localization in cultured feline uterine cells. Immunocytochemical (ICC) techniques were employed to visualize PR in mixed stromal and epithelial cell cultures treated with the phosphatase inhibitors, vanadate and okadaic acid, in the absence and presence of P. No significant effects on PR localization were observed. However, unexpected effects of the kinase inhibitor, staurosporine, on cellular integrity and PR localization were observed. Although inconsistent with results of a similar investigation on glucocorticoid receptor distribution, these data suggest a possible role of phosphorylation in steroid receptor localization.

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Edge Effects and the Reproductive Biology of the Clonal, Herbaceous, Perennial, Oxalis oregana : Pollen limitation, clonal reproduction, low seed set and implications for conservation
or
The Conceptualization of Self in a Monastic Population of Redwood Sorrel

Bruce B. Moreira

(Advisor: Karoly)

ABSTRACT

As forests become increasingly fragmented in the Pacific Northwest, a more complete understanding of the effects of forest fragmentation on the remaining ecosystem must be reached for the creation of informed and effective conservation strategies. This study examined the effect of fragment edge on the reproductive biology and morphology of the clonal, herbaceous perennial Oxalis oregana in the Mt. Hood National Forest, Oregon. This study also examined the genetic population structure of natural populations. Four paired sites were established along two clear-cuts and two roads in old-growth Douglas fir (Pseudotsuga menziesii) forests. Pollen supplementation and pollinator exclusion treatments were performed over the flowering season in spring, 1996 and size measurements were taken in May and September, 1996. Results from the floral manipulations indicated that there were no significant edge or site effects on pollinator limitation, as all of the plants set zero fruit. Floral phenology did show significant edge effects, but only in sites adjacent to clear-cuts. Population genetic information indicated a low amount of genetic variability for three of the four populations monitored, and a moderate amount for the fourth. The pollen manipulation, genetic population structure, and laboratory fruit set results allowed a number of hypotheses about the low natural fruit set to be addressed. This study indicated that low fruit set was probably a common phenomenon for O. oregana in the north end of the species range and that O. oregana might be useful as an indicator species for edge effects from fragmentation. Implications for species conservation were also discussed.

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Effects of Ultraviolet Radiation and Anthropogenic Acidification on Larval Fitness in the Asian Fire-Bellied Toad, Bombina orientalis

Jonathan Naito

(Advisor: Kaplan)

ABSTRACT

Recent papers on amphibian ecology have emphasized the role of environmental factors including pH, UV-B, and temperature as "stressors" that weaken immune systems leaving individuals more vulnerable to other agents. This suggests the need for research that examines the effects of interactions between several factors. The following laboratory study examined the main and interactive effects of pH and UV-B on embryonic mortality, morbidity, and development in the frog Bombina orientalis. When pH was analyzed as a main effect, pH and hatching morphology showed a non-linear relationship consistent with earlier research on the effects of acidity on larval morphology. However, when the three pH treatments were also irradiated by UV-B, time to hatching at pH 4.5, 5.0, and 6.0 was very similar. The significant effect of an interaction on a developmental measure known to be associated with larval fitness demonstrates the importance of considering the effects of synergistic interactions on larval morphology. Furthermore, the identical effects of UV-B irradiation at two different levels (ambient UV-B and a noticeably higher UV-B value) support the hypothesis that UV-B acts primarily as a stressor on developing amphibians, rather than having strong direct effects independently. This study was funded in part by a HHMI Undergraduate Research Program grant.

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Synaptic Modulation by Metabotropic Glutamate Receptors in the Reciprocal Dendrodendritic Synapses in the External Plexiform Layer of the Rat Olfactory Bulb

Michael Chi Hong Oh

(Advisors: Arch with Thomas Segerson, OHSU)

ABSTRACT

An important synapse in the olfactory bulb is the reciprocal dendrodendritic synapses formed between mitral and granule cell dendrites in the external plexiform layer. These synapses are unique in that neurotransmission occurs in both directions. Activation of a mitral cell by odor signals causes release of glutamate from the mitral cell secondary dendrites, depolarizing the surrounding granule cells. These granule cells then release GABA, resulting in feedback inhibition of the mitral cell. The activated granule cells can also inhibit other mitral cells nearby, resulting in lateral or surround inhibition. Interestingly, dendrodendritic synapses contain metabotropic glutamate receptors, G protein-coupled slow acting receptors, that may modulate neurotransmission at these synapses. Two important aspects of metabotropic glutamate receptors were studied in this project. First, possible protein-protein interactions between metabotropic glutamate receptors and synaptic proteins were investigated. Although it cannot be confirmed by the experiments performed in this study, these protein-protein interactions may be responsible for the localization of metabotropic glutamate receptors to specific synaptic domains. Second, synaptic modulatory function of metabotropic glutamate receptors in the dendrodendritic synapses was examined using isolated dendrodendritic synaptosomes. By applying superfusion to synaptosomal preparations, it is shown that activation of metabotropic glutamate receptors increases glutamatergic activity in the dendrodendritic synaptosomes. This study was funded in part by a HHMI Undergraduate Research Program grant.

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Synergistic effect of Stromal-Epithelial Interaction of Epidermal Growth Factor-Induced c-fos Expression

Kanao Otsu

(Advisor: McClellan)

ABSTRACT

Studies in ovariectomized mice showed that epidermal growth factor (EGF) replaced estradiol 17-ß (E2) in promoting growth of female reproductive tract tissues, but this effect required estrogen receptors and was inhibited by antiestrogens. Moreover, EGF, insulin-like growth factor 1 (IGF-1) and E2 acted synergistically to induce transcription of a reporter gene linked to a consensus estrogen response element, suggesting that combinations of growth factors and E2 may mediate estrogen action. The mRNA levels of c-fos, an immediate early gene and established marker of estrogen action, were examined in response to EGF, IGF-1, and E2 either alone and in combination in feline, oviductal stromal and epithelial cells. EGF, IGF or E2 induced small increases in c-fos expression and combinations of polypeptide growth factors and E2 appeared additive when stromal or epithelial cells were grown in isolation. Co-culturing stroma with epithelium resulted in a large increase in c-fos expression when cells were stimulated for 30 minutes with EGF alone whereas no combinatorial effects of agonists was detected. These data suggest that cell-cell interactions may play an important role that cannot be replaced by combinations of growth peptides thought to mediate estrogen action in the female reproductive tract. This study was funded in part by a HHMI Undergraduate Research Program grant.

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Kinetic Mechanism of Recombinant Saccharomyces cerevisiae Orotate Phosphoribosyltransferase Expressed in Escherichia coli

Raymond Tsou

(Advisor: McClard - BMB)

ABSTRACT

The kinetic mechanism of phosphoribosyltransferase (PRTase) has been suggested to proceed via either the random single-displacement kinetic mechanism or the bi-bi ping-pong kinetic mechanism. To study the kinetic mechanism of Saccharomyces cerevisiae orotate phosphoribosyltransferase (OPRTase), Dr. Ellis Adler, Dr. Ronald McClard, and John Notis, from Reed College, engineered the pREJ2 plasmid, which contains the S. cerevisiae URA5 gene, coding for OPRTase (EC 2.4.2.10). The pREJ2 plasmid was transformed into Escherichia coli strain CS101-4U1, and the overproduction system for S. cerevisiae OPRTase has been developed. The recombinant OPRTase was purified to homogeneity, and partial isotope exchange experiments between 32P-labeled inorganic pyrophosphate ([32P]PPi) and 5-phosphoribosyl-1-pyrophosphate (PRPP) were performed. The results showed that S. cerevisiae OPRTase did execute the partial isotope exchange reaction between [32P]PPi and PRPP. This study claims that S. cerevisiae OPRTase proceeds via a bi-bi ping-pong mechanism which is different from the random single-displacement mechanism of Salmonella typhimurium OPRTase (Bhatia et al., 1990). This study also postulates that the S. cerevisiae OPRTase, which can catalyze the reaction via the bi-bi ping-pong kinetics mechanism, might be a Mg2+-OPRTase complex.

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Biologically Realistic Single Neuron Modeling

Paul Wilkens

(Advisor: Arch)

ABSTRACT

This thesis examines the status of biologically realistic computational neuron models. The first chapter gives an extensive review of literature regarding mathematical descriptions of the elementary electrical and chemical interactions that occur in neurons. The second chapter examines the biological basis of neuron models, and details some modulatory behaviors which a realistic simulation might wish to incorporate. Finally, a biologically realistic model of a simple neuron, well-suited for network simulations, is presented and discussed.