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ArsZ from Ensifer adhaerens ST2 is a novel methylarsenite oxidase
Jun Zhang1, Yan-Ning Li1, Jian Chen2
1Jiangsu Key Laboratory for Organic Waste Utilization, Jiangsu Collaborative Innovation Center for Solid Organic Waste Resource Utilization, College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing, 210095, China.
Abstract:
Trivalent methylarsenite [MAs(III)] produced by biomethylation is more toxic than inorganic arsenite [As(III)]. Hence, MAs(III) has been proposed to be a primordial antibiotic. Other bacteria evolved mechanisms to detoxify MAs(III). In this study, the molecular mechanisms of MAs(III) resistance of Ensifer adhaerens ST2 were investigated. In the chromosome of E. adhaerens ST2 is a gene encoding a protein of unknown function. Here, we show that this gene, designated arsZ, encodes a novel MAs(III) oxidase that confers resistance by oxidizing highly toxic MAs(III) to relatively nontoxic MAs(V). Two other genes, arsRK, are adjacent to arsZ but are divergently encoded in the opposite direction. Heterologous expression of arsZ in Escherichia coli confers resistance to MAs(III) but not to As(III). Purified ArsZ catalyses thioredoxin- and NAPD+ -dependent oxidation of MAs(III). Mutational analysis of ArsZ suggests that Cys59 and Cys123 are involved in the oxidation of MAs(III). Expression of arsZ, arsR and arsK genes is induced by MAs(III) and As(III) and is likely controlled by the ArsR transcriptional repressor. These results demonstrate that ArsZ is a novel MAs(III) oxidase that contributes to E. adhaerens tolerance to environmental organoarsenicals. The arsZRK operon is widely present in bacteria within the Rhizobiaceae family.
Insights
Ensifer adhaerens ST2 possesses a novel enzyme, ArsZ, that detoxifies toxic methylarsenite [MAs(III)] by oxidizing it to less harmful methylarsenate [MAs(V)], conferring bacterial resistance to organoarsenicals.
Area of Science:
- Microbiology
- Environmental Science
- Biochemistry
Background:
- Trivalent methylarsenite [MAs(III)] is more toxic than inorganic arsenite [As(III)] and may have acted as a primordial antibiotic.
- Bacteria have developed detoxification mechanisms against MAs(III).
Purpose of the Study:
- To investigate the molecular mechanisms of MAs(III) resistance in Ensifer adhaerens ST2.
- To identify and characterize the function of a novel gene involved in MAs(III) detoxification.
Main Methods:
- Gene identification and characterization (arsZ).
- Heterologous expression of arsZ in Escherichia coli.
- Enzymatic assays of purified ArsZ protein.
- Mutational analysis of ArsZ.
- Gene expression analysis under arsenic stress.
Main Results:
- A novel gene, arsZ, encodes a MAs(III) oxidase that confers resistance to MAs(III) but not As(III).
- ArsZ catalyzes the oxidation of MAs(III) to MAs(V) using thioredoxin and NADP+.
- Specific cysteine residues (Cys59 and Cys123) in ArsZ are crucial for its enzymatic activity.
- Expression of arsZ, arsR, and arsK is induced by MAs(III) and As(III), likely regulated by ArsR.
- The arsZRK operon is prevalent in the Rhizobiaceae family.
Conclusions:
- ArsZ is a novel MAs(III) oxidase contributing to E. adhaerens ST2's tolerance to environmental organoarsenicals.
- The identified arsZRK operon represents a significant bacterial defense mechanism against toxic organoarsenicals.
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