Expression of the six chromate ion transporter homologues of Burkholderia xenovorans LB400

Abstract

The chromate ion transporter (CHR) superfamily comprises transporters that confer chromate resistance by extruding toxic chromate ions from cytoplasm. Burkholderia xenovorans strain LB400 has been reported to encode six CHR homologues in its multireplicon genome. We found that strain LB400 displays chromate-inducible resistance to chromate. Susceptibility tests of Escherichia coli strains transformed with cloned B. xenovorans chr genes indicated that the six genes confer chromate resistance, although under different growth conditions, and suggested that expression of chr genes is regulated by sulfate. Expression of chr genes was measured by quantitative reverse transcription-PCR (RT-qPCR) from total RNA of B. xenovorans LB400 grown under different concentrations of sulfate and exposed or not to chromate. The chr homologues displayed distinct expression levels, but showed no significant differences in transcription under the various sulfate concentrations tested, indicating that sulfate does not regulate chr gene expression in B. xenovorans. The chrA2 gene, encoded in the megaplasmid, was the only chr gene whose expression was induced by chromate and it was shown to constitute the chromate-responsive chrBACF operon. These data suggest that this determinant is mainly responsible for the B. xenovorans LB400 chromate resistance phenotype.

ChrASO, the chromate efflux pump of Shewanella oneidensis, improves chromate survival and reduction

“…Strikingly, expression of chrA SO in E . coli makes the strain capable of growing in the presence of high chromate concentrations, as also observed for the plasmid-encoded ChrA of Shewanella ANA-3 and for several ChrA of Burkholderia xenovorans LB400 [ 34 , 35 ]. Therefore ChrA SO most probably functions as a chromate efflux pump and expulses chromate ions from the cytoplasm, as demonstrated for some members of the CHR family [ 13 , 34 ].…”

“…In B . xenovorans LB400, which possesses six genes encoding chromate ion transporters, only the chrA2 gene present on a megaplasmid is subject to chromate-induced expression [ 35 ]. Interestingly, it was recently demonstrated that ChrA2 is the major determinant of chromate resistance in this organism [ 37 ].…”