Affiliation:
1. Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, Montana 59717
2. Department of Molecular Genetics, Biochemistry and Microbiology, University of Cincinnati College of Medicine, Cincinnati, Ohio 45267-0524
Abstract
ABSTRACT
Seminal regulatory controls of microbial arsenite [As(III)] oxidation are described in this study. Transposon mutagenesis of
Agrobacterium tumefaciens
identified genes essential for As(III) oxidation, including those coding for a two-component signal transduction pair. The transposon interrupted a response regulator gene (referred to as
aoxR
), which encodes an
ntrC
-like protein and is immediately downstream of a gene (
aoxS
) encoding a protein with primary structural features found in sensor histidine kinases. The structural genes for As(III) oxidase (
aoxAB
), a
c
-type cytochrome (
cytc
2
), and molybdopterin biosynthesis (
chlE
) were downstream of
aoxR
. The mutant could not be complemented by
aoxSR
in
trans
but was complemented by a clone containing
aoxS
-
aoxR
-
aoxA
-
aoxB
-
cytc
2
and consistent with reverse transcriptase (RT) PCR experiments, which demonstrated these genes are cotranscribed as an operon. Expression of
aoxAB
was monitored by RT-PCR and found to be up-regulated by the addition of As(III) to cell cultures. Expression of
aoxAB
was also controlled in a fashion consistent with quorum sensing in that (i) expression of
aoxAB
was absent in As(III)-unexposed early-log-phase cells but was observed in As(III)-unexposed, late-log-phase cells and (ii) treating As(III)-unexposed, early-log-phase cells with ethyl acetate extracts of As(III)-unexposed, late-log-phase culture supernatants also resulted in
aoxAB
induction. Under inducing conditions,
aoxS
expression was readily observed in the wild-type strain but significantly reduced in the mutant, indicating that AoxR is autoregulatory and at least partially controls the expression of the
aox
operon. In summary, regulation of
A. tumefaciens
As(III) oxidation is complex, apparently being controlled by As(III) exposure, a two-component signal transduction system, and quorum sensing.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology
Cited by
145 articles.
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