Multiple controls affect arsenite oxidase gene expression in Herminiimonas arsenicoxydans

Abstract

BackgroundBoth the speciation and toxicity of arsenic are affected by bacterial transformations, i.e. oxidation, reduction or methylation. These transformations have a major impact on environmental contamination and more particularly on arsenic contamination of drinking water. Herminiimonas arsenicoxydans has been isolated from an arsenic- contaminated environment and has developed various mechanisms for coping with arsenic, including the oxidation of As(III) to As(V) as a detoxification mechanism.ResultsIn the present study, a differential transcriptome analysis was used to identify genes, including arsenite oxidase encoding genes, involved in the response of H. arsenicoxydans to As(III). To get insight into the molecular mechanisms of this enzyme activity, a Tn5 transposon mutagenesis was performed. Transposon insertions resulting in a lack of arsenite oxidase activity disrupted aoxR and aoxS genes, showing that the aox operon transcription is regulated by the AoxRS two-component system. Remarkably, transposon insertions were also identified in rpoN coding for the alternative N sigma factor (σ54) of RNA polymerase and in dnaJ coding for the Hsp70 co-chaperone. Western blotting with anti-AoxB antibodies and quantitative RT-PCR experiments allowed us to demonstrate that the rpoN and dnaJ gene products are involved in the control of arsenite oxidase gene expression. Finally, the transcriptional start site of the aoxAB operon was determined using rapid amplification of cDNA ends (RACE) and a putative -12/-24 σ54-dependent promoter motif was identified upstream of aoxAB coding sequences.ConclusionThese results reveal the existence of novel molecular regulatory processes governing arsenite oxidase expression in H. arsenicoxydans. These data are summarized in a model that functionally integrates arsenite oxidation in the adaptive response to As(III) in this microorganism.

Involvement of RpoN in Regulating Bacterial Arsenite Oxidation

“…Second, the introduction of a precise deletion in a sequence that exhibited the well-known GG-(N10)-GC signature RpoN binding site (4) directly upstream of the aioB gene (but leaving the aioB translational start site intact) also resulted in the silencing of aioBA expression and As III oxidation. A recent random transposon-based mutation study with the organism Herminiimonas arsenicoxydans identified an rpoN::Tn5 mutant as being defective in As III oxidation (31). However, the necessary complementation experiments were not performed to confirm that the phenotype was actually due to the interrupted rpoN gene as opposed to transposon polar effects on the expression of rpoX (an rpoS-like sigma factor [61]) and other genes immediately downstream of what Duquesne et al (13) depicted previously as an H. arsenicoxydans operon that contains rpoN (31).…”

“…A recent random transposon-based mutation study with the organism Herminiimonas arsenicoxydans identified an rpoN::Tn5 mutant as being defective in As III oxidation (31). However, the necessary complementation experiments were not performed to confirm that the phenotype was actually due to the interrupted rpoN gene as opposed to transposon polar effects on the expression of rpoX (an rpoS-like sigma factor [61]) and other genes immediately downstream of what Duquesne et al (13) depicted previously as an H. arsenicoxydans operon that contains rpoN (31). The precisely targeted mutation and complementation experiments in the present study provide definitive evidence that the rpoN gene product is essential for aioBA expression and, hence, As III oxidation.…”

“…Subsequent follow-up characterizations of this organism and this process failed to materialize; however, approximately 2 decades later, Santini et al (52) described the isolation and initial characterization of a Rhizobiumlike bacterium (strain NT-26) that could grow chemolithoautotrophically with As III as a sole electron donor for energy generation and with CO 2 as a sole carbon source. Soon thereafter, and in part stimulated by the massive arsenic poisoning disaster in Bangladesh (2), a series of studies initiated the characterization of microbial As III oxidation in natural environments, including geothermal springs (9,11,12,17,19,24,25,35,51) and soils (41); in mining-contaminated environments (6,13,40); and, most recently, in anoxic photosynthesis (21,33 (28,31) indicated the role and importance of the sensor kinase AioS and its putative regulatory partner AioR (a bacterial enhancer binding protein), direct proof of these two proteins working together as part of a putative As III signal perception and transduction cascade was just recently provided by Sardiwal et al (54), who demonstrated the autophosphorylation of an AioS component and the AioS-specific phosphorylation of AioR. Recently, our work has expanded this regulatory model to now include a third component, AioX, which is a periplasmic As III binding protein that is also essential for aioBA expression (39 …”

Unified Nomenclature for Genes Involved in Prokaryotic Aerobic Arsenite Oxidation

“…However, the regulation mechanism has only been studied in a few of them (2,7,8,13). A complex mechanism for the expression of the structural genes for arsenite oxidase (aoxAB) involving quorum sensing as well as a two-component signal transduction system was described in Agrobacterium tumefaciens 5A (7).…”

“…Two-component regulatory genes aoxS and aoxR, located directly upstream of aoxAB, were identified, respectively, as a putative sensor histidine kinase and as a putative transcriptional regulator. The AoxSR system was also described in the heterotrophic bacterium Ochrobactrum tritici SCII24 (2) and H. arsenicoxydans (8), as well as in the chemolithoautotroph NT-26 (13). In the latter, it was designated AroSR (13).…”