posted on 2021-04-14, 13:36authored byKaixiang Shi, Manohar Radhakrishnan, Xingli Dai, Barry P. Rosen, Gejiao Wang
Synthetic
aromatic arsenicals such as roxarsone (Rox(V)) and nitarsone
(Nit(V)) have been used as animal growth enhancers and herbicides.
Microbes contribute to redox cycling between the relatively less toxic
pentavalent and highly toxic trivalent arsenicals. In this study,
we report the identification of nemRA operon from Enterobacter sp. Z1 and show that it is involved
in trivalent organoarsenical oxidation. Expression of nemA is induced by chromate (Cr(VI)), Rox(III), and Nit(III). Heterologous
expression of NemA in Escherichia coli confers resistance to Cr(VI), methylarsenite (MAs(III)), Rox(III),
and Nit(III). Purified NemA catalyzes simultaneous Cr(VI) reduction
and MAs(III)/Rox(III)/Nit(III) oxidation, and oxidation was enhanced
in the presence of Cr(VI). The results of electrophoretic mobility
shift assays and fluorescence assays demonstrate that the transcriptional
repressor, NemR, binds to either Rox(III) or Nit(III). NemR has three
conserved cysteine residues, Cys21, Cys106, and Cys116. Mutation of
any of the three resulted in loss of response to Rox(III)/Nit(III),
indicating that they form an Rox(III)/Nit(III) binding site. These
results show that NemA is a novel trivalent organoarsenical oxidase
that is regulated by the trivalent organoarsenical-selective repressor
NemR. This discovery expands our knowledge of the molecular mechanisms
of organoarsenical oxidation and provides a basis for studying the
redox coupling of environmental toxic compounds.