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The role of TonB-dependent receptors and alternate sigma factors in the Bradyrhizobium japonicum/soybean symbiosis.

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Call No.	QR82.R45 B38 2008
Author	Batchelet, Stephanie A.
Title	The role of TonB-dependent receptors and alternate sigma factors in the Bradyrhizobium japonicum/soybean symbiosis.
Date	2008
Abstract	Bradyrhizobium japonicum forms a nitrogen-fixing symbiosis with soybean. Signaling events, such as production of flavonoids and Nod factor, initiate the interaction, leading to formation of specialized root organs called nodules. A B. japonicum 61A152 mutant, fegA ::ΩStr, is blocked in plant cell invasion. fegA encodes an outer membrane receptor for the fungal siderophore, ferrichrome. fegB encodes an inner membrane protein of unknown function. The fegA ::ΩStr mutant is defective in Fe(III)-ferrichrome utilization. However, ferrichrome uptake is not required for the symbiosis, as suppressor mutants, unable to use ferrichrome, can be isolated from functional nodules. This suggests that FegA may bind a plant-signaling molecule. Consistent with this idea, FegA is predicted to belong to a class of receptors capable of signal transduction. Upon ligand binding, the N-terminal signaling domain of such receptors interacts with an inner membrane anti-sigma factor, leading to release of an extracytoplasmic function (ECF) sigma factor, to recruit RNA polymerase to target promoters. Regulated genes include those involved in iron uptake and virulence. We have shown that fegA is required for ferrichrome uptake and symbiosis, while loss of fegB has no affect on either function. Although the N-terminal domain of FegA is required for stability, leaving its function in signaling unresolved, mixed wild type 61A152 / fegA ::ΩStr infections result in partial complementation of fegA ::ΩStr, with mutants able to infect nodule cells. These results provide indirect evidence that the block in fegA ::ΩStr invasion is due to the inability of this mutant to signal to the plant. In order to further explore the signaling cascade involving FegA, we switched to using B. japonicum strain USDA110 whose complete genome sequence is now available. The closest fegA ortholog in USDA110, FhuA, encodes a ferrichrome receptor. Although FhuA is not involved in the symbiosis, we have identified an ECF sigma factor, Blr4928, which is required for the symbiosis. Furthermore, its presumptive anti-sigma factor, Blr4929, acts as a negative regulator of the symbiosis. Taken together, we have shown that the molecular details of B. japonicum 61A152 and USDA110 infection differ, and thus further comparative analyses will lead to insights into the B. japonicum /soybean symbiosis.
Description	xiv, 244 leaves : ill. (some col.) 28 cm.
Format	application/pdf
Advisor	Guerinot, Mary Lou O'Toole, George, Jr., Taylor, Ronald K., Helmann, John D.
Subject	Rhizobium -- Genetics. Cell receptors. Transcription factors. Bacteria.
College	Dartmouth College -- Department of Biological Sciences.
Issuing Body	Dartmouth Dissertations are made available through the Dartmouth Digital Library.
Degree	Thesis (Ph.D.)--Dartmouth College.
Copyright	All rights reserved. No part of this book may be reproduced in any form by any electronic or mechanical means without permission in writing from the author.
DOI	http://dx.doi.org/10.1349/ddlp.196

Title	Page 1
Format	application/pdf
Issuing Body	Dartmouth Dissertations are made available through the Dartmouth Digital Library.
Degree	Thesis (Ph.D.)--Dartmouth College.
Full Text	The Role of TonB-Dependent Receptors and Alternate Sigma Factors in the Bradyrhizobium japonicum/ Soybean Symbiosis A Thesis Submitted to the Faculty in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Biological Sciences by Stephanie A. Batchelet DARTMOUTH COLLEGE Hanover, New Hampshire May 28, 2008 Examining Committee: (Chairperson) Mary Lou Guerinot, Ph.D. George O’Toole, Jr., Ph.D. Ronald K. Taylor, Ph.D. John D. Helmann, Ph.D. Charles K. Barlowe, Ph.D. Dean of Graduate Studies

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