Signature Arsenic Detoxification Pathways in Halomonas sp. Strain GFAJ-1.

Shuangju Wu,Rui Gan,Tong Tong,Zhiqiang Li,Hao Bian,Shi Chen,Zixin Deng,Shiming Du,Lianrong Wang,Stephen J Giovannoni

doi:10.1128/mbio.00515-18

Shuangju Wu, Rui Gan + Show 8 more

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https://doi.org/10.1128/mbio.00515-18

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Abstract

ABSTRACTSince the original report that Halomonas sp. strain GFAJ-1 was capable of using arsenic instead of phosphorus to sustain growth, additional studies have been conducted, and GFAJ-1 is now considered a highly arsenic-resistant but phosphorus-dependent bacterium. However, the mechanisms supporting the extreme arsenic resistance of the GFAJ-1 strain remain unknown. In this study, we show that GFAJ-1 has multiple distinct arsenic resistance mechanisms. It lacks the genes to reduce arsenate, which is the essential step in the well-characterized resistance mechanism of arsenate reduction coupled to arsenite extrusion. Instead, GFAJ-1 has two arsenic resistance operons, arsH1-acr3-2-arsH2 and mfs1-mfs2-gapdh, enabling tolerance to high levels of arsenate. mfs2 and gapdh encode proteins homologous to Pseudomonas aeruginosa ArsJ and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), respectively, which constitute the equivalent of an As(V) efflux system to catalyze the transformation of inorganic arsenate to pentavalent organoarsenical 1-arseno-3-phosphoglycerate and its subsequent extrusion. Surprisingly, the arsH1-acr3-2-arsH2 operon seems to consist of typical arsenite resistance genes, but this operon is sufficient to confer both arsenite and arsenate resistance on Escherichia coli AW3110 even in the absence of arsenate reductase, suggesting a novel pathway of arsenic detoxification. The simultaneous occurrence of these two unusual detoxification mechanisms enables the adaptation of strain GFAJ-1 to the particularly arsenic-rich environment of Mono Lake.

Highlights

Since the original report that Halomonas sp. strain GFAJ-1 was capable of using arsenic instead of phosphorus to sustain growth, additional studies have been conducted, and GFAJ-1 is considered a highly arsenic-resistant but phosphorusdependent bacterium
It has not been proved that arsenic can be incorporated into biomolecules in strain GFAJ-1, the ability to thrive in 200 mM As(V) makes GFAJ-1 one of the most arsenic-resistant microorganisms described so far
A region highly resembling an “arsenic island” such as those found in other Halomonas species was immediately identified (Fig. 1) [27]. This arsenic island comprises a group of typical arsenic resistance determinants, a three-gene ars operon, a phosphate-specific transport operon, transporters of the major facilitator superfamily (MFS), and a putative glyceraldehyde-3-phosphate dehydrogenase (Fig. 2)

Summary

Introduction

Since the original report that Halomonas sp. strain GFAJ-1 was capable of using arsenic instead of phosphorus to sustain growth, additional studies have been conducted, and GFAJ-1 is considered a highly arsenic-resistant but phosphorusdependent bacterium. GFAJ-1 has two arsenic resistance operons, arsH1-acr3-2-arsH2 and mfs1-mfs2-gapdh, enabling tolerance to high levels of arsenate. Strain GFAJ-1 was previously reported to use arsenic as a substitute for phosphorus to sustain life under phosphate-limited conditions. This claim was later undermined by several groups, how GFAJ-1 can thrive in environments with high arsenic concentrations remains unclear. Ars is the most extensively studied arsenic detoxification system; the mechanism consists of (i) the uptake of arsenate by phosphate transporters and uptake of arsenite by aquaglyceroporins, (ii) transformation of As(V) to As(III) by arsenate reductases, and (iii) extrusion of As(III) by arsenite efflux permeases [9]. In addition to the above-mentioned two typical arsenic resistance operons, a broad diversity of ars operons has been characterized in different species [14,15,16,17,18]

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