Novel Pathway for Arsenic Detoxification in the Legume Symbiont
Sinorhizobium melilotiJournal Articles
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abstract
ABSTRACT
We report a novel pathway for arsenic detoxification in the legume symbiont
Sinorhizobium meliloti
. Although a majority of
ars
operons consist of three genes,
arsR
(transcriptional regulator),
arsB
[As(OH)
3
/H
+
antiporter], and
arsC
(arsenate reductase), the
S. meliloti ars
operon includes an aquaglyceroporin (
aqpS
) in place of
arsB
. The presence of AqpS in an arsenic resistance operon is interesting, since aquaglyceroporin channels have previously been shown to adventitiously facilitate uptake of arsenite into cells, rendering them sensitive to arsenite. To understand the role of
aqpS
in arsenic resistance,
S. meliloti aqpS
and
arsC
were disrupted individually. Disruption of
aqpS
resulted in increased tolerance to arsenite but not arsenate, while cells with an
arsC
disruption showed selective sensitivity to arsenate. The results of transport experiments in intact cells suggest that AqpS is the only protein of the
S. meliloti ars
operon that facilitates transport of arsenite. Coexpression of
S. meliloti aqpS
and
arsC
in a strain of
E. coli
lacking the
ars
operon complemented arsenate but not arsenite sensitivity. These results imply that, when
S. meliloti
is exposed to environmental arsenate, arsenate enters the cell through phosphate transport systems and is reduced to arsenite by ArsC. Internally generated arsenite flows out of the cell by downhill movement through AqpS. Thus, AqpS confers arsenate resistance together with ArsC-catalyzed reduction. This is the first report of an aquaglyceroporin with a physiological function in arsenic resistance.
