Arsenic is a metalloid that occurs naturally in aquatic and terrestrial environments. The high toxicity of arsenic derivatives converts this element in a serious problem of public health worldwide. There is a global arsenic geocycle in which microbes play a relevant role. Ancient exposure to arsenic derivatives, both inorganic and organic, has represented a selective pressure for microbes to evolve or acquire diverse arsenic resistance genetic systems. In addition, arsenic compounds appear to have been used as a toxin in chemical warfare for a long time selecting for an extended range of arsenic resistance determinants. Arsenic resistance strategies rely mainly on membrane transport pathways that extrude the toxic compounds from the cell cytoplasm. The ars operons, first discovered in bacterial R-factors almost 50 years ago, are the most common microbial arsenic resistance systems. Numerous ars operons, with a variety of genes and different combinations of them, populate the prokaryotic genomes, including their accessory plasmids, transposons, and genomic islands. Besides these canonical, widespread ars gene clusters, which confer resistance to the inorganic forms of arsenic, additional genes have been discovered recently, which broadens the spectrum of arsenic tolerance by detoxifying organic arsenic derivatives often used as toxins. This review summarizes the presence, distribution, organization, and redundance of arsenic resistance genes in prokaryotes.

The Great Oxidation Event resulted in integration of soft metals in a wide range of biochemical processes including, in our opinion, killing of bacteria by protozoa. Compared to pressure from anthropologic copper contamination, little is known on impacts of protozoan predation on maintenance of copper resistance determinants in bacteria. To evaluate the role of copper and other soft metals in predatory mechanisms of protozoa, we examined survival of bacteria mutated in different transition metal efflux or uptake systems in the social amoeba Dictyostelium discoideum. Our data demonstrated a strong correlation between the presence of copper/zinc efflux as well as iron/manganese uptake, and bacterial survival in amoebae. The growth of protozoa, in turn, was dependent on bacterial copper sensitivity. The phagocytosis of bacteria induced upregulation of Dictyostelium genes encoding the copper uptake transporter p80 and a triad of Cu(I)-translocating P -type ATPases. Accumulated Cu(I) in Dictyostelium was monitored using a copper biosensor bacterial strain. Altogether, our data demonstrate that Cu(I) is ultimately involved in protozoan predation of bacteria, supporting our hypothesis that protozoan grazing selected for the presence of copper resistance determinants for about two billion years.

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We report the draft genome sequence of arsenite-oxidizing Halomonas sp. strain HAL1, isolated from the soil of a gold mine. Genes encoding proteins involved in arsenic resistance and transformation, phosphate utilization and uptake, and betaine biosynthesis were identified. Their identification might help in understanding how arsenic and phosphate metabolism are intertwined. Halomonas sp. strain HAL1, which has a high level of tolerance to arsenite, was isolated from the soil of a gold mine in Daye County, Hubei Province, central China. Strain HAL1 is a heterotrophic, arsenite-oxidizing gammaproteobacterium under aerobic conditions. It is also moderately halophilic and can grow at NaCl concentrations ranging from 0.2 M to 2.0 M in LB, the optimal concentration being 0.8 M. Interest in arsenic and phosphate metabolism in Halomonas was sparked by a recent controversial report claiming that arsenate could replace phosphate in DNA (6, 10). It was therefore desirable to obtain the genomic sequence of a Halomonas strain able to survive in extremely low phosphate concentrations in the presence of arsenic.The genome of Halomonas sp. strain HAL1 was sequenced using a 454 GS FLX sequencer (3) and was assembled using GS de novo assembler ("Newbler"), version 2.5.3. The assembled contigs were submitted to the RAST annotation server for subsystem classification and functional annotation (1). The protein-coding genes (CDSs) were assigned using BLASTp with the KEGG orthology (KO) database. GC content was calculated using an in-house Perl script. The NCBI Prokaryotic Genomes Automatic Annotation Pipeline (PGAAP; http://www.ncbi.nlm.nih.gov/genomes /static/Pipeline.html) was employed for gene annotation in preparation for submission to GenBank.The draft genome sequence of Halomonas sp. strain HAL1 comprises 4,347,024 bases at 36-fold coverage. The assembled genome consists of 89 large contigs (Ͼ500 bp) with an average contig size of 102,049 bp and a GϩC content of 54.1%. The draft genome sequence contains 4,082 CDSs, 54 tRNAs, and 8 rRNAs. For the CDSs, 3,439 proteins could be assigned to Cluster of Orthologous Groups (COG) families (9). One thousand nine hundred fifty-four proteins have orthologs (bit score of Ͼ60) with the five reference strains, Halomonas elongata and four others, Chromohalobacter salexigens DSM 3043, Hahella chejuensis KCTC 2396, Cellvibrio japonicas Ueda107, and Pseudomonas entomophila L48, identified by RAST as the closest neighbors to HAL1.The Halomonas sp. strain HAL1 genome carries multiple genes potentially involved in arsenic resistance. There are two arsenic resistance operons containing genes encoding ArsC, ArsH, and ACR3 but only one operon with a gene encoding ArsR. One of these operons is adjacent to two genes, aioA and aioB, that encode the enzyme arsenite oxidase (4, 7). There is also a pst operon in the immediate vicinity of this arsenic cluster that might play a role in integrating phosphate and arsenic metabolism. In addition, there is another pst operon on the chromosome. Furthermore, a num...