Co-expression of Cyanobacterial Genes for Arsenic Methylation and Demethylation inEscherichia coliOffers Insights into Arsenic Resistance.
Arsenite [As(III)] and methylarsenite [MAs(III)] are the most toxic inorganic and methylated arsenicals, respectively. As(III) and MAs(III) can be interconverted in the unicellular cyanobacteriumNostocsp. PCC 7120 (Nostoc), which has both thearsMgene (NsarsM), which is responsible for arsenic methylation, and thearsIgene (NsarsI), which is responsible for MAs(III) demethylation. It is not clear how the cells prevent a futile cycle of methylation and demethylation. To investigate the relationship between arsenic methylation and demethylation, we constructed strains ofEscherichia coliAW3110 (ΔarsRBC) expressingNsarsMor/andNsarsI. Expression ofNsarsIconferred MAs(III) resistance through MAs(III) demethylation. Compared to NsArsI, NsArsM conferred higher resistance to As(III) and lower resistance to MAs(III) by methylating both As(III) and MAs(III). The major species found in solution was dimethylarsenate [DMAs(V)]. Co-expression ofNsarsMandNsarsIconferred As(III) resistance at levels similar to that withNsarsMalone, although the main species found in solution after As(III) biotransformation was methylarsenate [MAs(V)] rather than DMAs(V). Co-expression ofNsarsMandNsarsIconferred a higher level of resistance to MAs(III) than found with expression ofNsarsMalone but lower than expression of onlyNsarsI.Cells co-expressing both genes converted MAs(III) to a mixture of As(III) and DMAs(V). InNostoc NsarsMis constitutively expressed, whileNsarsIis inducible by either As(III) or MAs(III). Thus, our results suggest that at low concentrations of arsenic, NsArsM activity predominates, while NsArsI activity predominates at high concentrations. We propose that coexistence ofarsMandarsIgenes inNostoccould be advantageous for several reasons. First, it confers a broader spectrum of resistance to both As(III) and MAs(III). Second, at low concentrations of arsenic, the MAs(III) produced by NsArsM will possibly have antibiotic-like properties and give the organism a competitive advantage. Finally, these results shed light on the role of cyanobacteria in the arsenic biogeochemical cycle.
Key Lab of Urban Environment and Health, Institute of Urban Environment - Chinese Academy of SciencesXiamen, China; University of Chinese Academy of SciencesBeijing, China.
Yu
Y
Yan
Key Lab of Urban Environment and Health, Institute of Urban Environment - Chinese Academy of Sciences Xiamen, China.
Xi-Mei
XM
Xue
Key Lab of Urban Environment and Health, Institute of Urban Environment - Chinese Academy of SciencesXiamen, China; Fujian Provincial Key Laboratory of Agroecological Processing and Safety Monitoring, College of Life Sciences, Fujian Agriculture and Forestry UniversityFuzhou, China.
Yu-Qing
YQ
Guo
Key Lab of Urban Environment and Health, Institute of Urban Environment - Chinese Academy of SciencesXiamen, China; State Key Lab of Urban and Regional Ecology, Research Center for Eco-environmental Sciences - Chinese Academy of SciencesBeijing, China.
Yong-Guan
YG
Zhu
Key Lab of Urban Environment and Health, Institute of Urban Environment - Chinese Academy of Sciences Xiamen, China.
Jun
J
Ye
MAs(III) antibiotic; Nostoc sp. PCC 7120; arsenic demethylation; arsenic methylation; arsenic resistance