AUTHOR=Yan Yu , Xue Xi-Mei , Guo Yu-Qing , Zhu Yong-Guan , Ye Jun 

TITLE=Co-expression of Cyanobacterial Genes for Arsenic Methylation and Demethylation in Escherichia coli Offers Insights into Arsenic Resistance

JOURNAL=Frontiers in Microbiology

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2017.00060

DOI=10.3389/fmicb.2017.00060

ISSN=1664-302X

ABSTRACT=<p>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 cyanobacterium <italic>Nostoc</italic> sp. PCC 7120 (<italic>Nostoc</italic>), which has both the <italic>arsM</italic> gene (<italic>NsarsM</italic>), which is responsible for arsenic methylation, and the <italic>arsI</italic> gene (<italic>NsarsI</italic>), 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 of <italic>Escherichia coli</italic> AW3110 (Δ<italic>arsRBC</italic>) expressing <italic>NsarsM</italic> or/and <italic>NsarsI</italic>. Expression of <italic>NsarsI</italic> conferred 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 of <italic>NsarsM</italic> and <italic>NsarsI</italic> conferred As(III) resistance at levels similar to that with <italic>NsarsM</italic> alone, although the main species found in solution after As(III) biotransformation was methylarsenate [MAs(V)] rather than DMAs(V). Co-expression of <italic>NsarsM</italic> and <italic>NsarsI</italic> conferred a higher level of resistance to MAs(III) than found with expression of <italic>NsarsM</italic> alone but lower than expression of only <italic>NsarsI.</italic> Cells co-expressing both genes converted MAs(III) to a mixture of As(III) and DMAs(V). In <italic>Nostoc NsarsM</italic> is constitutively expressed, while <italic>NsarsI</italic> is 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 of <italic>arsM</italic> and <italic>arsI</italic> genes in <italic>Nostoc</italic> could 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.</p>