Rhodococcus, a species of bacteria commonly found in the soil, is an under-explored producer of small bioactive compounds including siderophores, pigments and antibiotics. MTM3W5.2 is a strain of Rhodococcus that was previously discovered to produce an antibiotic-like compound that has inhibitory effects on other Rhodococcus strains, including the veterinary pathogen, R. equi. The biosynthetic gene cluster responsible for production of the antibiotic has been identified, and a small gene, BTZ20_3964 at the start of the operon is believed to be a luxR solo regulator of the gene cluster. The goal of this project was to determine this gene’s status as a regulator for the gene cluster. Merodiploids were constructed using the deletion construct, pEX18Km3964AD to obtain a double crossover recombination event to replace the functional gene with the deletion construct. However, evidence indicates that an illegitimate recombination event occurred to produce a merodiploid strain.

The quorum-sensing response of Vibrio fischeri involves a complex network of genes (encoding regulatory proteins as well as sRNAs), that govern host-association and production of bioluminescence. A key regulator of this system is LuxR, which is the transcriptional activator of the lux operon as well as several other genes in. LuxR also autoregulates its own transcription, which we have shown causes bistability and hysteresis in the quorum-sensing response. This behavior allows the system to maintain a stable and robust response in the face of environmental fluctuation or decreases in external autoinducer concentration caused by other sources. There are many factors that are known to regulate luxR expression, including the ArcA redox-responsive regulator, the cAMP-CRP secondary metabolism regulator, and components of the quorum-sensing pathway like LitR. Because of this, LuxR levels are critical in both the timing of quorum-sensing induction, as well as the maintenance of the response over time. This makes it a potential target for multiple levels of regulation in response to factors such as environmental and metabolic conditions, as well as other components of the quorum-sensing network. Another important global regulatory protein in V. fischeri (and most other species of Gram-negative proteobacteria) is the post-transcriptional regulator CsrA. CsrA controls processes involved in carbon storage and utilization, as well as the transition from exponential to stationary phase growth. We have demonstrated that CsrA is regulated by two sRNAs (CsrB1 and CsrB2) in V. fischeri. Because CsrA regulates changes in cell behavior and is an important metabolic regulator, there is a good possibility that it has some interactions with the quorum-sensing regulon, whose endproduct, bioluminescence, creates a large metabolic demand from the cell. In an effort to determine at which point in the quorum-sensing regulatory network CsrA regulation is important, epistasis experiments were designed using factorial design, which is a subset of statistical analysis of variance (ANOVA). This method was used to generate a high degree of confidence in the data, so that even minor interactions in the regulatory networks could be established. By altering the levels of CsrA expression in various mutant strains of V. fischeri, we have demonstrated that CsrA acts by an unknown mechanism to increase the transcription of luxR when the quorum-sensing regulator LitR is absent. Our results also demonstrated that CsrA mediates this effect through repression of ArcA activity, which is known to act directly on the luxR and luxI intergenic region as a repressor. This indicates that CsrA may bypass the upstream parts of the quorum-sensing regulatory cascade that lead to litR activation, so that LitR and LuxR may be regulated differently in response to certain conditions. This work has shown that the interactions between global regulons can coordinately control the amount of quorum-sensing induction by affecting the level of LuxR in the cell. The balance of these regulatory networks allows the cell to tightly regulate the quorum-sensing response. Thus, LuxR serves as a critical regulatory hub in the cell, at which multiple signals can be integrated in order to generate the appropriate cellular response. / Ph. D.

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. / Ph. D.

The LuxR protein family is a class of quorum-sensing regulated bacterial transcription factors that alter gene expression as a function of ligand detection. This coincides with a high population density and/or a low rate of signal ligand diffusion. The majority of LuxR proteins are activated only in the presence of the signal ligand, an acyl-homoserine lactone (AHL). EsaR, from the corn pathogen Pantoea stewartii, represents a subset of LuxR homologues that are active in the absence of AHL and deactivated by its presence. The mechanism by which EsaR responds to AHL in a manner opposite to that of the majority of LuxR homologues remains elusive. Unlike the majority of LuxR homologues, which require AHL for purification, EsaR can be purified and biochemically investigated in the absence and presence of AHL. This work sought to answer questions regarding the structure-function relationship of the LuxR homologue, EsaR. Fluorescence anisotropy was used to determine the relative DNA-binding affinity of wild type EsaR and three AHL-independent EsaR variants in the presence and absence of AHL. This enabled for quantitative analysis of the relative binding affinities of these AHL-independent variants for the EsaR binding site, the esa box. The results demonstrate that one AHL-independent EsaR variant has a slightly higher affinity for the esa box in the presence, rather than the absence of AHL. The affinity of the other two for the DNA is not impacted by AHL, potentially due to an inability to transduce the signal of ligand detection to the DNA binding domain. Constructs containing only the EsaR N-terminal domain (NTD) were also developed. These constructs circumvented solubility issues associated with the full-length protein, allowing for additional biochemical analysis. It was determined that the EsaR NTD alone is sufficient for multimerization and ligand binding. Additionally, preliminary X-ray crystallography efforts have established some of the early parameters required to solve the crystal structure of the EsaR ligand binding domain in both the presence and absence of AHL. If pursued, these structures would be the first solved of a LuxR homologue ligand binding domain in both the presence and absence of the native AHL, potentially demonstrating the conformational change that occurs as a result of ligand binding. Collectively, these findings have established some of the groundwork required to resolve the question of what sort of conformational changes occur in EsaR as a result of ligand binding. / Master of Science

The LuxR family of proteins serves as quorum-sensing transcriptional regulators in proteobacteria. At high population densities, a small acyl-homoserine lactone (AHL) molecule, produced by a LuxI homologue, accumulates in the environment. The LuxR proteins bind to their respective AHL when the ligand accumulates to sufficient levels. Once bound to AHL, the holoproteins usually become functional as transcriptional activators. However, there is a subset of LuxR homologues, the EsaR subfamily, which is active without the AHL ligand and becomes inactivated once bound to it. EsaR is the best understood member of this subfamily. It controls virulence in the corn pathogen Pantoea stewartii ssp. stewartii. Solubility issues have previously limited structural studies of LuxR homologues as the proteins could not be purified without the AHL ligand. A soluble recombinant EsaR protein, HMGE, is biologically active and can be purified in the absence and presence of AHL, unlike most other LuxR homologues. Using HMGE, amino acid substitutions and Förster resonance energy transfer (FRET), experimental methods were designed for determining the dimerization interface of EsaR and for testing the hypothesis that EsaR undergoes a conformational shift when presented with the AHL ligand. To identify residues of the dimerization interface, heterodimerization assays were designed, involving either coexpression or coincubation of wild-type EsaR and variant HMGE proteins. In this assay, the inability of the proteins to copurify by nickel affinity chromatography would indicate that the modified residue(s) are important for dimerization of EsaR. To determine the conformational change that EsaR undergoes when bound to the AHL ligand, a FRET assay was developed to estimate the distances between amino acid residues in the absence and presence of AHL. Future work will have to include a few modifications to the methods and/or control experiments. This study provides the basis upon which the present methods can be further developed and later used for structural studies of EsaR. / Master of Science

Vibrio vulnificus is a model environmental organism exhibiting a classical starvation response during nutrient limitation as well as a non-culturable state when exposed to low temperatures. In addition to these classic global responses, this organism is an opportunistic pathogen that exhibits numerous virulence factors. This organism was chosen as the model organism for the identification of regulators of the viable but nonculturable response (VBNC) and the starvation-induced maintenance of culturability (SIMC) that occurs when cells are starved prior to low temperature incubation. In order to accomplish this, three indirect approaches were used; proteomics, investigation of intercellular signalling pathways and genetic analysis of regulators involved in these responses. Two-dimensional gel electrophoresis was used to identify proteins expressed under conditions that induced SIMC. It was determined that carbon and long-term phosphorus starvation were important in the SIMC response. V. vulnificus was shown to possess genes, luxS and smcR, that are homologues of genes involved in signalling system system 2 in Vibrio harveyi. Signal molecules were produced upon starvation and the entry to stationary phase in V. vulnificus. Furthermore, a null mutation in smcR, a transcriptional regulator was shown to have pleiotropic effects in V. vulnificus, including up-regulation of numerous virulence factors and a defect in starvation survival and development of the SIMC response. We propose that V. vulnificus possesses a signalling system analogous to that of system 2 in V. harveyi, and that this system is involved in the regulation of stationary phase and starvation adaptation in this organism.

Quorum sensing (QS) cell-cell communication systems are utilized by bacteria to coordinate their behaviour according to cell density. Several different types of QS signal molecules have been identified, among which acyl-homoserine lactones (AHLs) produced by Proteobacteria have been studied to the greatest extent. QS has been shown to be involved in many aspects of bacterial life, including virulence, bioluminescence, symbiosis, antibiotic production, swarming and swimming motility, biofilm formation, conjugation and growth inhibition. Although QS has been studied extensively in cultured microorganisms, little is known about the QS systems of uncultured microorganisms and the roles of these systems in microbial communities. To extend our knowledge of QS systems and to better understand the signalling that takes place in the natural environment, in the first part of this thesis, isolation and characterization of new QS systems from metagenomic libraries constructed using DNA from activated sludge and soil were described. Using an Agrobacterium biosensor strain, three cosmids (QS6-1, QS10-1 and QS10-2) that encode the production of QS signals were identified and DNA sequence analysis revealed that all three clones encode a novel luxI family AHL synthase and a luxR family transcriptional regulator. Thin layer chromatography revealed that these LuxI homolog proteins are able to synthesize multiple AHL signals. Tandem mass spectrometry analysis revealed that LuxIQS6-1 directs the synthesis of at least three AHLs, 3-O-C14:1 HSL, 3-O-C16:1 HSL and 3-O-C14 HSL; LuxIQS10-1 directs the synthesis of at least 3-O-C12 HSL and 3-O-C14 HSL; while LuxIQS10-2 directs the synthesis of at least C8 HSL and C10 HSL. Two possible new AHLs, C14:3 HSL and (?)-hydroxymethyl-3-O-C14 HSL, were also found to be synthesized by LuxIQS6-1. Agrobacterium tumefaciens is a plant pathogen that causes crown gall disease. Its ability to transfer and integrate foreign DNA into plant genome also makes it a useful tool for plant genetic engineering. Ethylene, the gaseous plant hormone, has been reported to be important for both crown gall development and A. tumefaciens mediated transformation efficiency to plants. ACC deaminase, an enzyme that can break down ACC, the direct precursor of ethylene biosynthesis in plants, is a mechanism used by some plant growth promoting bacteria (PGPB) to promote plant growth by reducing stress ethylene levels. In the second part of this thesis, the effect of ACC deaminase on A. tumefaciens induced crown gall development and on A. tumefaciens mediated transformation efficiency was studied. By either introduction of an ACC deaminase encoding gene into the virulent strain A. tumefaciens C58 or co-inoculation of A. tumefaciens C58 with an ACC deaminase containing PGPB P. putida UW4, using different plant systems including tomato plants and castor bean plants, it was found that the presence of an ACC deaminase significantly inhibited crown gall development. It was also found that introduction of an acdS gene into the disarmed A. tumefaciens strain GV3101::pMP90 reduced the ethylene levels evolved by plants during infection and cocultivation process and increased the transformation efficiency of commercialized canola cultivars. The A. tumefaciens D3 strain was reported to contain an ACC deaminase encoding gene (acdS). In this study it was determined that this strain is an avirulent strain and shows plant growth promoting activity. When co-inoculated with A. tumefaciens C58 on castor bean stems, both the wild type and the acdS knockout mutant showed biocontrol activity and were able to significantly inhibit crown gall formation, with the wild type strain showing slightly better tumor inhibition effects. The mutation of acdS and its regulatory gene lrpL in A. tumefaciens D3 was also found to affect QS signal production of this strain, which indicates a cross talk between the two sets of genes.

Quorum sensing (QS) cell-cell communication systems are utilized by bacteria to coordinate their behaviour according to cell density. Several different types of QS signal molecules have been identified, among which acyl-homoserine lactones (AHLs) produced by Proteobacteria have been studied to the greatest extent. QS has been shown to be involved in many aspects of bacterial life, including virulence, bioluminescence, symbiosis, antibiotic production, swarming and swimming motility, biofilm formation, conjugation and growth inhibition. Although QS has been studied extensively in cultured microorganisms, little is known about the QS systems of uncultured microorganisms and the roles of these systems in microbial communities. To extend our knowledge of QS systems and to better understand the signalling that takes place in the natural environment, in the first part of this thesis, isolation and characterization of new QS systems from metagenomic libraries constructed using DNA from activated sludge and soil were described. Using an Agrobacterium biosensor strain, three cosmids (QS6-1, QS10-1 and QS10-2) that encode the production of QS signals were identified and DNA sequence analysis revealed that all three clones encode a novel luxI family AHL synthase and a luxR family transcriptional regulator. Thin layer chromatography revealed that these LuxI homolog proteins are able to synthesize multiple AHL signals. Tandem mass spectrometry analysis revealed that LuxIQS6-1 directs the synthesis of at least three AHLs, 3-O-C14:1 HSL, 3-O-C16:1 HSL and 3-O-C14 HSL; LuxIQS10-1 directs the synthesis of at least 3-O-C12 HSL and 3-O-C14 HSL; while LuxIQS10-2 directs the synthesis of at least C8 HSL and C10 HSL. Two possible new AHLs, C14:3 HSL and (?)-hydroxymethyl-3-O-C14 HSL, were also found to be synthesized by LuxIQS6-1. Agrobacterium tumefaciens is a plant pathogen that causes crown gall disease. Its ability to transfer and integrate foreign DNA into plant genome also makes it a useful tool for plant genetic engineering. Ethylene, the gaseous plant hormone, has been reported to be important for both crown gall development and A. tumefaciens mediated transformation efficiency to plants. ACC deaminase, an enzyme that can break down ACC, the direct precursor of ethylene biosynthesis in plants, is a mechanism used by some plant growth promoting bacteria (PGPB) to promote plant growth by reducing stress ethylene levels. In the second part of this thesis, the effect of ACC deaminase on A. tumefaciens induced crown gall development and on A. tumefaciens mediated transformation efficiency was studied. By either introduction of an ACC deaminase encoding gene into the virulent strain A. tumefaciens C58 or co-inoculation of A. tumefaciens C58 with an ACC deaminase containing PGPB P. putida UW4, using different plant systems including tomato plants and castor bean plants, it was found that the presence of an ACC deaminase significantly inhibited crown gall development. It was also found that introduction of an acdS gene into the disarmed A. tumefaciens strain GV3101::pMP90 reduced the ethylene levels evolved by plants during infection and cocultivation process and increased the transformation efficiency of commercialized canola cultivars. The A. tumefaciens D3 strain was reported to contain an ACC deaminase encoding gene (acdS). In this study it was determined that this strain is an avirulent strain and shows plant growth promoting activity. When co-inoculated with A. tumefaciens C58 on castor bean stems, both the wild type and the acdS knockout mutant showed biocontrol activity and were able to significantly inhibit crown gall formation, with the wild type strain showing slightly better tumor inhibition effects. The mutation of acdS and its regulatory gene lrpL in A. tumefaciens D3 was also found to affect QS signal production of this strain, which indicates a cross talk between the two sets of genes.

Quorum sensing is a mechanism used by many proteobacteria to regulate expression of target genes in a population-dependent manner. The quorum sensing system of Vibrio fischeri activates the luminescence (lux) operon when the autoinducer signaling molecule (N-3-oxohexanoyl homoserine lactone) is recognized and bound by the activator protein LuxR. LuxR subsequently binds to the lux box centered at à 42.5 bp upstream of the transcription initiation site and activates transcription from the lux operon promoter, resulting in the emission of light at high cell densities. LuxR consists of 250 amino acids arranged into an N-terminal (regulatory) domain and a C-terminal (activation) domain, and is thought to function as an ambidextrous activator capable of making multiple contacts with the alpha and sigma subunits of RNA polymerase (RNAP). Published work describing the results of alanine scanning mutagenesis performed on the C-terminal domain of LuxR (residues 190-250) has identified residues (K198, W201 and I206) that appear to play a role in positive control of transcription initiation. Additional mutagenesis of residues 180-189 has been undertaken via a three-primer or four-primer PCR-based method in this study. Variants of LuxR were screened for their ability to activate luciferase production and to repress transcription from an artificial promoter, and production of full-length LuxR was measured, in an attempt to identify additional positive control variants. No additional positive control variants were found in this study. Work has also been undertaken to identify intergenic suppressors between positive control variants of LuxR and the RNAP alpha subunit, RpoA. Starting with a recombinant Escherichia coli strain encoding the lux operon and LuxR variant I206E, a random chemical mutagenesis was performed on a vector encoding RpoA. Following transformation of the mutated plasmids encoding RpoA, high throughput luminescence assays were used to identify isolates with phenotypes brighter than the control. Isolation of an intergenic suppressor will confirm the existence of protein-protein interactions between LuxR and RpoA within the transcription initiation complex. The ability of other LuxR family members to establish productive protein-protein interactions with RNAP necessary for transcription initiation was also examined. LuxR homologues EsaR of Pantoea stewarti ssp. stewartii, a repressor of known function, and ExpR of Erwinia carotovora subsp. carotovora were also analyzed for their ability to activate the lux operon, as well as to repress transcription from an artificial promoter containing the lux box. / Master of Science

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