Two bacterial enhancer binding proteins LuxO and SypG activate expression of a small RNA Qrr1 in the beneficial microbe Vibrio fischeri
Open Access
- Author:
- Surrett, Ericka
- Graduate Program:
- Biochemistry, Microbiology, and Molecular Biology (PHD)
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 02, 2022
- Committee Members:
- Charles Fisher, Committee Member & Related Areas Repres
Shaun Mahony, Committee Member & Major Field Represnt
Timothy Miyashiro, Chair & Major Field Represnt
Timothy Meredith, Committee Member & Major Field Represnt
Sarah Ades, Committee Member & Major Field Represnt
James Ferry, Committee Member & Major Field Represnt
Ken Keiler, Professor in Charge/Director of Graduate Studies - Keywords:
- Vibrio fischeri
bacterial enhancer binding proteins
Euprymna scolopes
squid
transcription factors
signaling networks
quorum sensing
LuxO
SypG
Qrr1
small regulatory RNA
transcription activation
bEBP
BinK
symbiosis
beneficial microbe
sigma-54
Vibrionaceae
RscS - Abstract:
- Many bacteria and eukaryotes can establish long-term, intimate associations, which are referred to as symbioses. The process by which symbiotic bacteria colonize, grow, and express specific symbiotic traits within a host is called symbiosis establishment. In many cases, the bacterial symbionts are also called beneficial bacteria because they express traits that confer fitness advantages to their eukaryotic hosts. Beneficial bacteria perform many functions from synthesizing essential vitamins within the human gut to inhibiting colonization by pathogenic microbes on epithelial surfaces. Because beneficial bacteria improve host physiology, it is important to increase understanding of how these microbes establish symbiosis with a host. The outcomes of such studies have the potential to facilitate the development of therapeutics and prebiotics that promote colonization by beneficial bacteria. The symbiotic relationship between the Hawaiian bobtail squid, Euprymna scolopes, and the bioluminescent bacterium Vibrio fischeri offers a platform to study the molecular mechanisms that enable host-colonization by a beneficial microbe. V. fischeri inhabits the squid light organ and produces light via bioluminescence, which aids the squid in avoiding detection by prey. V. fischeri is horizontally transmitted, which means that the squid acquires the bacteria from an environmental reservoir. To successfully establish symbiosis with its host, V. fischeri must express specific traits that permit light organ colonization. The expression of these traits is regulated at the level of transcription by a special class of proteins called transcription factors. The alternative sigma factor σ54 is a transcription factor that allows bacteria to rapidly respond to changes in the environment by activating transcription of specific genes. A specialized type of transcription factor called a bacterial enhancer binding protein, or bEBP, is required for σ54-dependent transcriptional activation. In V. fischeri, σ54 is absolutely required for cells to establish light organ symbiosis, suggesting there are also bEBPs that are implicated in this process. Currently, there exists a gap in knowledge regarding the ways in which bEBPs function in V. fischeri to permit symbiosis establishment. The work presented in this dissertation addresses this knowledge gap by increasing understanding how two bEBPs regulate expression of a small RNA Qrr1 which promotes symbiosis establishment. The experimental results provided here demonstrate that Qrr1 promotes symbiosis establishment by facilitating entry into the light organ. This work provides insight into how Qrr1 expression is regulated during the initial stages of light organ colonization. LuxO is a bEBP that activates expression of Qrr1, which is a small, regulatory RNA that post-transcriptionally inhibits the master regulator, LitR. LuxO activity is inhibited by quorum sensing, which is the process that describes how bacteria synthesize and detect of small, signaling molecules called autoinducers. As the number of cells within a population increases, so does the ambient concentration of autoinducer such that quorum sensing is a way for cells to gauge cell density. Autoinducers alter the activity of signaling networks in ways that effect changes in gene expression, which leads to the coordinated expression of specific traits, such as bioluminescence and motility, within a bacterial population. Because autoinducer concentrations increase with cell density, Qrr1 is typically not expressed in populations of V. fischeri that have reached a quorum. However, this work revealed that an additional bEBP, SypG, leads to increased qrr1 transcriptional activity in a LuxO-independent manner. LuxO and SypG are homologs that share similar structures that suggest they depend on the same regulatory mechanisms. LuxO is active when V. fischeri are free-living in seawater, whereas SypG is active during the initial stages of host-colonization under conditions that are predicted to inhibit LuxO activity. V. fischeri has adapted to maintain these two homologous bEBPs as their overlapping functions are compartmentalized to different stages of the symbiont’s lifecycle. These findings have expanded the model regarding how Qrr1 is transcriptionally regulated in V. fischeri. V. fischeri is a member of the Fischeri clade within the Vibrionaceae family. The Vibrionaceae family consists of a diverse group of bacteria that include free-living, beneficial, and pathogenic microbes. This work also demonstrated that approximately half of the taxa within the Vibrionaceae encode homologs of LuxO and SypG and a least one qrr gene, which highlights the possibility that Qrr1 transcription is also activated by both bEBPs in other microbes. In addition, this work revealed that the fish pathogen Aliivibrio salmonicida, which is another member of the Fischeri clade, encodes a SypG homolog that can also induce Pqrr1 activity from the promoter of its own qrr1 gene. As Qrrs regulate symbiotic traits in other Vibrios, these finding suggests SypG and LuxO activation of qrr among the Vibrionaceae and warrants further investigation. Altogether, these studies have increased understanding of ways in which host-associated microbes utilize bEBPs to establish symbiosis.