Affiliation:
1. Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania, USA
Abstract
ABSTRACT
Monosaccharides capable of serving as nutrients for the soil bacterium
Agrobacterium tumefaciens
are also inducers of the
vir
regulon present in the tumor-inducing (Ti) plasmid of this plant pathogen. One such monosaccharide is galacturonate, the predominant monomer of pectin found in plant cell walls. This ligand is recognized by the periplasmic sugar binding protein ChvE, which interacts with the VirA histidine kinase that controls
vir
gene expression. Although ChvE is also a member of the ChvE-MmsAB ABC transporter involved in the utilization of many neutral sugars, it is not involved in galacturonate utilization. In this study, a putative tripartite ATP-independent periplasmic (TRAP) transporter, GaaPQM, is shown to be essential for the utilization of galacturonic acid; we show that residue R169 in the predicted sugar binding site of the GaaP is required for activity. The gene upstream of
gaaPQM
(
gaaR
) encodes a member of the GntR family of regulators. GaaR is shown to repress the expression of
gaaPQM
, and the repression is relieved in the presence of the substrate for GaaPQM. Moreover, GaaR is shown to bind putative promoter regions in the sequences required for galacturonic acid utilization. Finally,
A. tumefaciens
strains carrying a deletion of
gaaPQM
are more sensitive to galacturonate as an inducer of
vir
gene expression, while the overexpression of
gaaPQM
results in strains being less sensitive to this
vir
inducer. This supports a model in which transporter activity is crucial in ensuring that
vir
gene expression occurs only at sites of high ligand concentration, such as those at a plant wound site.
Funder
NSF | BIO | Division of Integrative Organismal Systems
Publisher
American Society for Microbiology
Subject
Ecology,Applied Microbiology and Biotechnology,Food Science,Biotechnology
Cited by
10 articles.
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