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<oai_dc:dc xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:oai_dc="http://www.openarchives.org/OAI/2.0/oai_dc/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/oai_dc/ http://www.openarchives.org/OAI/2.0/oai_dc.xsd">
  <dc:contributor>Shelley E. Hoeft</dc:contributor>
  <dc:contributor>Ronald S. Oremland</dc:contributor>
  <dc:contributor>Chad W. Saltikov</dc:contributor>
  <dc:creator>Kamrun Zargar</dc:creator>
  <dc:date>2010</dc:date>
  <dc:description>&lt;div id="abstract-1" class="section abstract"&gt;&lt;p id="p-1"&gt;Although arsenic is highly toxic to most organisms, certain prokaryotes are known to grow on and respire toxic metalloids of arsenic (i.e., arsenate and arsenite). Two enzymes are known to be required for this arsenic-based metabolism: (i) the arsenate respiratory reductase (ArrA) and (ii) arsenite oxidase (AoxB). Both catalytic enzymes contain molybdopterin cofactors and form distinct phylogenetic clades (ArrA and AoxB) within the dimethyl sulfoxide (DMSO) reductase family of enzymes. Here we report on the genetic identification of a “new” type of arsenite oxidase that fills a phylogenetic gap between the ArrA and AoxB clades of arsenic metabolic enzymes. This “new” arsenite oxidase is referred to as ArxA and was identified in the genome sequence of the Mono Lake isolate&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;Alkalilimnicola ehrlichii&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;MLHE-1, a chemolithoautotroph that can couple arsenite oxidation to nitrate reduction. A genetic system was developed for MLHE-1 and used to show that&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;arxA&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;(gene locus ID&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;mlg&lt;/i&gt;_&lt;i&gt;0216&lt;/i&gt;) was required for chemoautotrophic arsenite oxidation. Transcription analysis also showed that&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;mlg&lt;/i&gt;_&lt;i&gt;0216&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;was only expressed under anaerobic conditions in the presence of arsenite. The&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;mlg&lt;/i&gt;_&lt;i&gt;0216&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;gene is referred to as&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;arxA&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;because of its greater homology to&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;arrA&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;relative to&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;aoxB&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;and previous reports that implicated Mlg_0216 (ArxA) of MLHE-1 in reversible arsenite oxidation and arsenate reduction&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;in vitro&lt;/i&gt;. Our results and past observations support the position that ArxA is a distinct clade within the DMSO reductase family of proteins. These results raise further questions about the evolutionary relationships between arsenite oxidases (AoxB) and arsenate respiratory reductases (ArrA).&lt;/p&gt;&lt;/div&gt;&lt;p id="p-2"&gt;Arsenic is toxic to most organisms and is known to cause cancer in humans. However, bacteria have adapted several biotransformation pathways that function to either couple the reduction or oxidation of arsenicals to energy conservation and growth (1). The enzymologies of these two pathways have several features in common. The arsenate respiratory reductase (ArrAB) and arsenite oxidase (AoxAB) enzymes are usually composed of at least two subunits, a small iron-sulfur cluster-containing subunit (ArrB and AoxA) and a larger molybdopterin-containing catalytic subunit (ArrA and AoxB). Although they catalyze arsenic redox chemistry, ArrA and AoxB form distinct phylogenetic clades within the dimethyl sulfoxide (DMSO) reductase family of molybdenum-containing enzymes (16,&lt;span&gt;&amp;nbsp;&lt;/span&gt;24).&lt;/p&gt;&lt;p id="p-3"&gt;Culture-dependent approaches have resulted in the isolation of a variety of diverse bacteria that metabolize arsenic (reviewed in reference&lt;span&gt;&amp;nbsp;&lt;/span&gt;26). Many of these isolates have had their genomes sequenced, which has been insightful for understanding the composition and diversity of&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;arr&lt;/i&gt;and&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;aox&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;gene clusters. In the arsenite-oxidizing nitrate reducer&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;Alkalilimnicola ehrlichii&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;strain MLHE-1 (a haloalkaliphile isolated from Mono Lake [CA]) (10,&lt;span&gt;&amp;nbsp;&lt;/span&gt;15), bioinformatic analysis of its genome revealed the absence of genes homologous to the arsenite oxidase genes of the&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;aoxB&lt;/i&gt;type. Instead, two genes (&lt;i&gt;mlg&lt;/i&gt;_&lt;i&gt;0216&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;and&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;mlg&lt;/i&gt;_&lt;i&gt;2426&lt;/i&gt;) were identified that better resembled the catalytic subunit of the arsenate respiratory reductase (20); however, MLHE-1 has not been shown to respire (or reduce) arsenate (15). Recent work by Richey et al. (20) showed that the Mlg_0216 protein (and not Mlg_2426) was expressed under chemolithoautotrophic (10 mM arsenite and 10 mM nitrate) growth conditions. Moreover, it was shown that Mlg_0216 exhibits both arsenate reductase and arsenite oxidase activities&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;in vitro&lt;/i&gt;. These observations raised the question, is the&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;mlg&lt;/i&gt;_&lt;i&gt;0216&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;gene required for arsenite oxidation&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;in vivo&lt;/i&gt;? In this report, we addressed this question by developing a genetic system in MLHE-1, generating strains with mutations in&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;mlg&lt;/i&gt;_&lt;i&gt;0216&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;and&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;mlg&lt;/i&gt;_&lt;i&gt;2426&lt;/i&gt;, and physiologically characterizing the resulting strains. Our results implicate&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;mlg&lt;/i&gt;_&lt;i&gt;0216&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;in chemolithoautotrophic arsenite oxidation coupled to nitrate respiration.&lt;/p&gt;</dc:description>
  <dc:format>application/pdf</dc:format>
  <dc:identifier>10.1128/JB.00244-10</dc:identifier>
  <dc:language>en</dc:language>
  <dc:publisher>American Society for Microbiology</dc:publisher>
  <dc:title>Identification of a novel arsenite oxidase gene, arxA, in the haloalkaliphilic, arsenite-oxidizing bacterium alkalilimnicola ehrlichii strain MLHE-1</dc:title>
  <dc:type>article</dc:type>
</oai_dc:dc>