The Phosphate Versus Arsenate Debate
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bs_bs_banner Highlight Indispensable or toxic? The phosphate versus arsenate debate M. José Huertas1 and Carmen Michán2* arsenic may substitute phosphorous to allow growth. 1Instituto de Bioquímica Vegetal y Fotosíntesis, Centro GFAJ-1 was isolated from highly contaminated sediments de Investigaciones Isla de la Cartuja, Universidad de and was able to grow in arsenate solution (at 60% the rate Sevilla-CSIC, Av. Américo Vespucio 49, 41092, Seville, of growth in phosphate supplemented medium). Without Spain. arsenate or phosphate the strain was unable grow. 2Universidad de Córdoba, Campus de Rabanales, Dept. Thereafter, a number of published works have reported of Biochemistry and Molecular Biology, Edificio Severo contradictory results to those of Wolfe-Simon et al. Last Ochoa C-6, 2aPlanta, 14071, Córdoba, Spain. July, Science published two independent reports on the subject. The work performed by Professor Vorholt’s group Arsenic (As) is toxic, carcinogenic and causes serious (Erb et al., 2012) used highly pure reactives combined health problems. While As occurs naturally due to vol- elegantly with state-of-the-art chemical tracing techniques canic activity, the major anthropogenic sources of As are to monitor phosphate substitution by arsenate. The find- metal processing, burning of coal and arsenic-based ings showed that phosphate is absolutely necessary for 3- the growth of GFAJ-1, suggesting that previous results pesticides or herbicides. Arsenate (AsO4 ) and arsenite 3- were misleading due to the presence of impurities in the (AsO3 ) are the primary chemical forms found in soil. Because of the wide distribution of arsenic compounds, chemicals used. Although arsenate was found to be com- arsenic resistance is widespread among living organisms bined to biomolecules as sugars or acids in the cultures, (Nordstrom, 2002). Most resistance systems reduce arse- they demonstrate that these products are generated abi- nate to arsenite and sequester it in a vacuole or expel it otically. Furthermore, the authors found no evidence of from the cell (Stolz et al., 2006). Intensive research has arsenate incorporation into DNA or other core metabolic shed light on dissimilatory As (V) reduction and As (III) molecules. Similar conclusions were drawn by Dr Red- oxidation pathways. field’s group (Reaves et al., 2012). Their findings showed Phosphate is essential to life. Living beings need phos- that the GFAJ-1 strain grows in the complete absence of phorus to function, along with other elements such as arsenate and that phosphate is absolutely necessary. hydrogen, oxygen, carbon, nitrogen and sulfur. The phos- Again, chemical impurities are named as the cause of 3- conflicting results of the original GFAJ-1 report (Wolfe- phate ion, PO4 , plays several essential cellular roles, such as maintaining the structure of DNA and RNA, com- Simon et al., 2011). Dr Redfield’s group also investigated bining with lipids to build cell membranes and transporting the putative presence of arsenate in DNA, while doing so energy within the cell through the molecule adenosine using an alternative experimental method. The authors triphosphate (ATP). postulated that arsenate incorporation in DNA would During the last two years, the metabolic antagonism result in higher instability. They electrophoretically tested that exists between arsenic and phosphorus in microbial hydrolysis of chromosomal DNA isolated from GFAJ-1 metabolism has been the focus of intense study. Wolfe- cultures grown under phosphate or arsenate and were Simon and colleagues (2011) revolutionized this area of unable to detect any significant amount of As in the research by describing the isolation of a bacterial strain, nucleic acids. Consequently arsenate does not appear to GFAJ-1, capable of thriving in a growth medium contain- act as a replacement for phosphate in DNA within this ing arsenate and lacking phosphate, hypothesizing that strain. Taken together, these results, while not as exceptional as originally thought, still position GFAJ-1 as a strain of great value by virtue of its ability to tolerate arsenate. *For correspondence. E-mail [email protected]; Tel. (+34) 957 Thus, the mechanism through which this is achieved rep- 218082; Fax (+34) 957 218688. resents a highly worthwhile area of study. Microbial Biotechnology (2013) 6(3), 209–211 Of most importance to this topic is the question of how doi:10.1111/1751-7915.12024 Funding Information No funding information provided. the GFAJ-1 strain is able to glean enough phosphate in © 2012 The Authors. Microbial Biotechnology published by Blackwell Publishing Ltd and Society for Applied Microbiology. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. 210 M. J. Huertas and C. Michán the arsenate-rich environment from which it was originally A key limitation of the above studies is the uncertain isolated. A mechanistic explanation had been published in purity of the chemicals used. This uncertainty leads to Nature (Elias et al., 2012) where the authors studied a set the possibility that cross-contamination between arsenite of phosphate-binding proteins (PBP) that are part of the and phosphate may have occurred. As arsenic and phos- transport chain bacteria use to uptake phosphate from the phorous are very close in the periodic table, separating environment. The ability of different PBPs to discriminate them chemically is not easy. On the other hand, in between phosphate and arsenate was investigated, complex natural environments, chemical methods cannot finding that most had a 500- to 850-fold higher selectivity measure other limiting factors such as bioavailability, and for phosphate. Even more impressive, GFAJ-1’s PBP-2, therefore future studies should aim to use biologically which is upregulated in low-phosphate conditions, based measurement methods for this purpose. One showed a 4500-fold selectivity for phosphate over arse- possible biological system for arsenite detection was nate. The observed differences in phosphate selectivity reported in Microbial Biotechnology (Wackwitz et al., are related to the binding of anions to these proteins, 2008). The manuscript describes the construction of dif- which form a dense and rigid network of ion-dipole inter- ferent recombinant strains using the ArsR sensor from actions. This network is sensitive to geometric changes. Escherichia coli to control transcription from two very dif- The authors found, through X-ray crystallography, that ferent reporter genes: the well-known lacZ coding for there are differences in alignment of one of the hydrogen b-galactosidase and the Saccharomyces cerevisiae cyto- bond angles in the binding pocket when PBPs bind arse- chrome peroxidase ccp gene. Combination of both nate, allowing them to select phosphate over arsenate reporters provides an extremely easy and fast method even in highly arsenate-rich environments. for semi-quantitative detection of As with a threshold as Additionally, Basturea and colleagues (2012) provide a low as 4 mgl-1. This system represents an excellent simple explanation for how phosphorous may be obtained model for the development of biosensors capable of to in arsenate-rich or phosphate-limiting environments. They detecting rare elements both in laboratory solutions and show that, under these conditions, ribosome degradation in the environment. is induced, which releases free bases from RNA and A final related topic that deserves more detailed study provides sufficient phosphate to allow limited growth. are the microbial processes that, together with inorganic Together with arsenate versus phosphate discrimination and physical mechanisms, contribute to the global arsenic and the ability to obtain phosphorus, it would be expected cycle. The environmental factors that drive arsenic mobi- that a resistance mechanism preventing the intracellular lization and the influence of microbial As metabolism in accumulation of As would be important for the survival of the environment should be a key focus of future studies. the GFAJ-1 strain in high-arsenate and phosphate-limiting conditions. In studies that relate to this, Lopez-Maury and colleagues (2009) constructed a Synechocystis mutant in Conflict of interest which all genes encoding arsenate reductases were inac- None declared. tivated. The strain was more sensitive to arsenate in an arsenate-rich medium devoid of phosphate than in a medium with phosphate (where the mutant grows similarly References to wild type), suggesting that small amounts of As (V) enter Basturea, G.N., Harris, T.K., and Deutscher, M.P. (2012) the cells under P-limiting conditions. Growth of a bacterium that apparently uses arsenic instead A twist in the phosphate versus arsenate debate was of phosphorus is a consequence of massive ribosome brought to the forefront in recent Environmental Microbi- breakdown. J Biol Chem 287: 28816–28819. ology article. In Agrobacterium tumefaciens, a model Elias, M., Wellner, A., Goldin-Azulay, K., Chabriere, E., microorganism used to study the genes involved in As (III) Vorholt, J.A., Erb, T.J., and Tawfik, D.S. (2012) The molecular basis of phosphate discrimination in arsenate- oxidation, genes related to arsenate oxidation (aioBA and rich environments. Nature 491: 134–137. aioSR) are adjacent to genes involved in phosphorous Erb, T.J., Kiefer, P., Hattendorf, B., Gunther, D., and Vorholt, acquisition (pst/pho) (Kang et al., 2012), suggesting that