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Journal of Bacteriology, November 1998, p. 5739-5748, Vol. 180, No. 21
Department of Biological Sciences, Howard
Hughes Medical Institute, Stanford University, Stanford, California
94305-5020
Received 8 June 1998/Accepted 28 August 1998
Rhizobium meliloti can occupy at least two distinct
ecological niches; it is found in the soil as a free-living saprophyte, and it also lives as a nitrogen-fixing intracellular symbiont in root
nodules of alfalfa and related legumes. One approach to understanding
how R. meliloti alters its physiology in order to become an
integral part of a developing nodule is to identify and characterize
genes that are differentially expressed by bacteria living inside
nodules. We used a screen to identify genes under the control of the
R. meliloti regulatory protein NodD3, SyrM, or SyrA. These
regulatory proteins are expressed by bacteria growing inside the root
nodule. One gene isolated in this screen was mapped to pSymB and
displayed complex regulation. The gene was downregulated by the
syrA gene product and also by glucose and succinate. This gene, referred to as agpA, encodes a periplasmic binding
protein that is most similar to proteins from the periplasmic
oligopeptide binding protein family. It is likely that AgpA binds
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
-Galactoside Uptake in Rhizobium meliloti:
Isolation and Characterization of agpA, a Gene Encoding a
Periplasmic Binding Protein Required for Melibiose and
Raffinose Utilization
-galactosides, because
-galactosides induce the expression of
agpA, and agpA mutants cannot utilize or
transport these sugars. Activity of an
agpA::TnphoA fusion was downregulated
by SyrA. Because syrA is known to be expressed at high
levels in intracellular symbiotic R. meliloti and at
low levels in the free-living bacteria, we propose that AgpA may belong
to the class of gene products whose expression decreases when R. meliloti becomes an intracellular symbiont.
*
Corresponding author. Present address: University of
Connecticut, Department of Molecular and Cell Biology, 75 N. Eagleville Rd., U-44, Storrs, CT 06269. Phone: (860) 486-5923. Fax: (860) 486-1784. E-mail: Gage{at}uconnvm.uconn.edu.
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