Existence of a robust haloacid transport system in a Burkholderia species bacterium

Bacterium Burkholderia sp. MBA4 can utilize haloacids as the sole carbon and energy source for growth. We have previously reported that a haloacid operon, encoding for a dehalogenase (Deh4a) and an associated permease (Deh4p), was responsible for the transformation and uptake of haloacids in MBA4. A disruption of deh4p in MBA4 caused a decrease in monochloroacetate (MCA) uptake, confirming its role as a haloacid transporter. However, this disruptant retained 68% of its MCA-uptake activity indicating the possibility of an alternative system. In this study, we report the identification of a second MCA-inducible haloacid transporter (Dehp2) in MBA4. Its function was confirmed by gene disruption and heterologous expression in Escherichia coli. A dehp2− mutant has 30% less, and an E. coli expressing Dehp2 has 40% more, of wildtype MCA-uptake activity. Quantitative RT-PCR illustrated that the minor loss of MCA-uptake activity in single disruptants of deh4p and dehp2 was partly due to a compensatory expression of the alternative gene. Competition assay and kinetics study revealed that Deh4p has a higher affinity for MCA while Dehp2 prefers chloropropionate. A deh4p−dehp2− double mutant retained 36% of MCA-uptake activity, indicating a robustness of the haloacid uptake systems. The MCA uptake activities mediated by Deh4p, Dehp2 and the uncharacterized system were completely abolished by protonophore carbonyl cyanide 3-chlorophenylhydrazone, suggesting that transmembrane electrochemical gradient is the driving force for MCA uptake.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (168 K)Download as PowerPoint slideHighlights► A second haloacid transporter Dehp2 was identified in Burkholderia sp. MBA4. ► Deh4p and Dehp2 are major haloacid transporters in MBA4. ► Deh4p and Dehp2 have different substrate specificities. ► Transmembrane electrochemical gradient is the driving force for MCA transport.