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Chapter 11 : Regulation of Arsenic Metabolic Pathways in Prokaryotes
Category: Applied and Industrial Microbiology; Environmental Microbiology
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Despite the toxicity of arsenic to most organisms, certain prokaryotes have the ability to grow on arsenic oxyanions by using them as alternative electron acceptors or electron donors. The common forms of arsenic used for these metabolic pathways are arsenate and arsenite. The end result of arsenic-based metabolism is that the organism couples oxidation/reduction of arsenite/arsenate to growth and/or arsenic resistance. Several reviews have been published on the subject of microbial arsenate respiration and oxidation. This chapter presents a summary of the genes and enzymes for arsenate respiration and arsenite oxidation. The chemical differences between arsenate and arsenite manifest different biological responses. In contrast to numerous single-pathway investigations, very few proteomic and transcriptomic approaches have been used for investigating global or systems level changes to the biology of arsenic-metabolizing prokaryote. A variety of microbial ecology studies have been carried out to investigate the abundance, distribution, and expression of ars, arr, and aox genes in a variety of environments. Microbial ecology studies have resulted in an extensive database of arr and aox sequences from a variety of environments. Molecular tools are now available to carry out quantitative aox and arr gene expression studies in environmental samples. These approaches could be used, for example, to determine the occurrence of “hot spots” for biologically mediated arsenate reduction. This could help identify zones in a groundwater environment that are more prone to arsenic mobilization.
Arsenic metabolic pathways. (A) Biochemical models for arsenite oxidation coupled to oxygen respiration; detoxifying reduction of arsenate by ArsC; the uptake of arsenate by the Pit, phosphate inorganic transporter; arsenite efflux by ArsB; and anaerobic respiration of arsenate by ArrAB. The regulatory pathways are also included for arsenite oxidation (AoxXSR), detoxifying arsenate reduction (ArsR), and arsenate respiratory reduction (ArsR and ArrTSR). Instead of ArrC, Shewanella sp. ANA-3 uses the membrane-bound tetraheme cytochrome CymA for transporting electrons from menaquinol to ArrAB. In other bacteria, there is genomic evidence that a transmembrane protein, ArrC, carries out this function. (B) Genomic regions of several bacteria that have arr gene clusters. The arrSR genes code for a two-component histidine kinase sensor and response regulators; arrT codes for a putative periplasmic phosphonate binding protein. The methyl-accepting chemotaxis-like protein gene of Wolinella codes for a putative chemotaxis protein. (C) Genomic regions of several bacteria that have aox gene clusters. Similar to arrSR, the aoxSR genes code for a two-component histidine kinase sensor and response regulators. Both ArrT and AoxX are predicted to bind either arsenite or arsenate in the periplasm and activate ArrS and AoxS phosphorylation, respectively. Activation of ArrS and AoxS would phosphorylate the cognate response regulators ArrR and AoxR to bind promoter regions of arr and aox operons, resulting in activation of transcription. The Shewanella species that have arr genes do not appear to have the arrC and arrTSR genes. The ArsR2 repressor mediates regulation of arr in Shewanella sp. ANA-3. arr transcription is activated in the presence of arsenite. 10.1128/9781555817190.ch11.f1
Phylogenetic analysis of arsenate respiratory reductases (ArrA), AoxB-type arsenite oxidases, and the ArxA-type arsenite oxidase of MLHE-1. The unrooted tree was constructed using a neighbor-joining method; gaps were ignored in the final phylogeny. The numbering refers to representative amino acid sequences ArrA and AoxB as described below. Arsenate respiratory reductase group (ArrA): 1, Shewanella piezotolerans (WP3, YP_002311519); 2, Shewanella sp. ANA-3 (*AAQ01672); 3, Chrysiogenes AAU11839 *; 4, Geobacter lovleyi ZP_ 01593421; 5, Geobacter uraniireducens Rf4 ZP_01140714; 6, Bacillus selenitireducens str. MLS10 AAQ19491*; 7, Bacillus arseniciselenatis str. E1H AAU11841*; 8, Sulfurospirillum barnesii str. SES-3 AAU11840*; 9, Wolinella succinogenes NP_906980*; 10, Desulfosporosinus ABB02056*; 11, Desulfitobacterium ZP_01372404 *; 12, MLMS-1 ZP_01288668 *; 13, Desulfonatronospira thiodismutans ASO3-1 ZP_03737819; 14, Natranaerobius thermophilus YP_ 001916826; 15, Halarsenatibacter silvermanii SLAS-1 ACF74513 *; 16, Alkaliphilus metalliredigens ZP_00800578; 17, Alkaliphilus oremlandii OhILAs ZP_01360543 *. AoxB arsenite oxidase group: 18, NT26 AAR05656 **; 19, Agrobacterium tumefaciens ABB51928 **; 20, Ochrobactrum tritici ACK38267; 21, Xanthobacter autotrophicus Py2 ZP_01198801; 22, Nitrobacter hamburgensis YP_571843; 23, Roseovarius sp. 217 ZP_01034989; 24, Alcaligenes AAQ19838 **; 25, Ralstonia sp. 22 ACX69823; 26, Herminiimonas arsenicoxydans AAN05581 **; 27, Burkholderia multivorans ZP 0157266830; 28, Rhodoferax ferrireducens YP_524325; 29, Thiomonas sp. 3As CAM58792; 30, Pseudomonas sp. TS44 ACB05943; 31, Halomonas sp. HAL1 ACF77048; 32, Chloroflexus aurantiacus ZP_00356; 33, Thermus thermophilus YP_145366 **. The asterisks * and ** indicate that the organism is known to respire arsenate or to oxidize arsenite, respectively. TMAO, trimethylamine N-oxide. 10.1128/9781555817190.ch11.f2