Affiliation:
1. Division of Plant Sciences, University of Missouri—Columbia, 108 Waters Hall, Columbia, Missouri 65211
Abstract
ABSTRACT
Pseudomonas syringae
pv. tomato strain DC3000, a pathogen of tomato and
Arabidopsis
, occurs as an epiphyte. It produces
N
-acyl homoserine lactones (AHLs) which apparently function as quorum-sensing signals. A Tn
5
insertion mutant of DC3000, designated PsrA
−
(Psr is for
Pseudomonas
sigma regulator), overexpresses
psyR
(a LuxR-type regulator of
psyI
) and
psyI
(the gene for AHL synthase), and it produces a ca. 8-fold-higher level of AHL than does DC3000. The mutant is impaired in its ability to elicit the hypersensitive reaction and is attenuated in its virulence in tomato. These phenotypes correlate with reduced expression of
hrpL
, the gene for an alternate sigma factor, as well as several
hrp
and
hop
genes during early stages of incubation in a Hrp-inducing medium. PsrA also positively controls
rpoS
, the gene for an alternate sigma factor known to control various stress responses. By contrast, PsrA negatively regulates
rsmA1
, an RNA-binding protein gene known to function as negative regulator, and
aefR
, a
tetR
-like gene known to control AHL production and epiphytic fitness in
P. syringae
pv. syringae. Gel mobility shift assays and other lines of evidence demonstrate a direct interaction of PsrA protein with
rpoS
promoter DNA and
aefR
operator DNA. In addition, PsrA negatively autoregulates and binds the
psrA
operator. In an AefR
−
mutant, the expression of
psyR
and
psyI
and AHL production are lower than those in DC3000, the AefR
+
parent. In an RpoS
−
mutant, on the other hand, the levels of AHL and transcripts of
psyR
and
psyI
are much higher than those in the RpoS
+
parent, DC3000. We present evidence, albeit indirect, that the RpoS effect occurs via
psyR
. Thus, AefR positively regulates AHL production, whereas RpoS has a strong negative effect. We show that AefR and RpoS do not regulate PsrA and that the PsrA effect on AHL production is exerted via its cumulative, but independent, effects on both AefR and RpoS.
Publisher
American Society for Microbiology
Subject
Ecology,Applied Microbiology and Biotechnology,Food Science,Biotechnology
Cited by
59 articles.
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