GFAJ-1

Normal growth of GFAJ-1 on medium containing phosphate

GFAJ-1 is an extremophilic strain of bacteria isolated from Mono Lake in central California . On December 2, 2010, the discoverers reported at a NASA press conference that the bacterium was able to incorporate arsenic instead of phosphorus into biomolecules , especially its DNA . As a result, various other scientists questioned whether this assumption is justified on the basis of the available data. A review in 2012 showed that arsenic has no part in the genetic information of the bacterial strain and that the thesis must therefore be rejected. A gatekeeper protein is discussed as the cause of the metabolic activity of arsenic in GFAJ-1, which controls the uptake of phosphorus salts with a selectivity of 4500 to 1 compared to arsenic salts.

discovery

The Mono Lake requires special adaptations of organisms because of its high salt and arsenic content. Fish cannot survive there.

The Mono Lake was chosen as the starting point for the search for extremophiles, because the lake is very rich in arsenic and therefore an occurrence of microorganisms adapted to these conditions seemed likely there. Samples taken from the lake were cultivated on nutrient media with increasing arsenate concentrations and the strain GFAJ-1 was isolated therefrom. The work was carried out by a group of scientists at various research institutes in the USA on behalf of NASA . Coordinates and finances the project from the NASA Astrobiology Institute at was Ames Research Center , the researchers themselves came from the US Geological Survey , the Arizona State University , the Lawrence Livermore National Laboratory , the Duquesne University and the Stanford Synchrotron Radiation Center of SLAC . Felisa Wolfe-Simon was in charge .

features

According to sequence analyzes of the 16S rRNA, GFAJ-1 is a Gammaproteobacterium from the Halomonadaceae family . It can grow in aerobic culture at a pH of 9.8 with glucose as a carbon source, vitamins and trace elements at 40 mmol / l arsenate and around 3 μmol / l phosphate , even if the growth is faster in the presence of higher phosphate concentrations. In the presence of arsenate, the bacteria grow in the form of rods with a length of about two micrometers, a diameter of about one micrometer and about one and a half times the volume of bacteria grown on a nutrient medium containing phosphate, which could be due to the formation of a vacuole- like structure in the cells.

According to the analysis of the 16S rRNA sequences, GFAJ-1 is closely related to other moderately halophilic (salt-loving) bacteria of the Halomonadaceae family. In a cladogram, the strain is placed in the midst of various members of the Halomonas genus , including H. alkaliphila and H. venusta , although it has not been explicitly assigned to this genus.

Escherichia coli strain O157: H7     Halomonas alkaliphila     Halomonas venusta strain NBSL13     GFAJ-1     Halomonas sp. GTW     Halomonas sp. G27     Halomonas sp. DH77     Halomonas sp. mp3     Halomonas sp. IB-O18     Halomonas sp. ML-185

Discussion about the incorporation of arsenic in biomolecules

GFAJ-1 on medium containing arsenate

Chemical analyzes have shown that bacteria grown in nutrient medium containing arsenate and almost free of phosphate still contain very small amounts of phosphate. Arsenic, on the other hand, could be found in different fractions of the bacteria in more precise analyzes , especially in the fractions containing nucleic acids and proteins . Also, by mass spectrometry ( Nano Sims ) arsenic was detected in fractions containing genomic DNA and RNA of the bacteria contained. Wolfe-Simon et al. interpreted this as the incorporation of arsenic into the nucleic acids, without, however, providing direct evidence of the incorporation of arsenic into the bacterial DNA. In this case, arsenic would not only have been tolerated in high concentrations, but GFAJ-1 could also incorporate it into macromolecules as a phosphate substitute in the event of a phosphate deficiency, even if this is accompanied by reduced growth. The authors of the study described this as the first evidence for an alternative biochemistry in which different elements are used in the basic building blocks of a cell than in other living things. These finds and their interpretation generated a great deal of interest in the media, also because some scientists emphasized the possible importance of such a discovery for astrobiology , since the previously only theoretically assumed possibility of such biochemical flexibility also for the definition of the goals in the search for possible extraterrestrials Life matters.

Several biochemists and microbiologists publicly expressed doubts about the results of the study shortly after the NASA press conference and the subsequent publication in Science . The criticism is primarily aimed at a faulty measurement setup, possible measurement errors, overinterpretation of the data, a lack of understanding of the chemical processes and the assumption that arsenates are incorporated into the DNA. According to the critics, the traces of phosphate in the selection media, mentioned by the authors of the original publication themselves, could have been sufficient for a normal phosphate balance of the bacteria. In addition, bonds in DNA actually formed with arsenate would necessarily have been destroyed in the test method used, so that the strands found must be completely normal, phosphate-based DNA. The arsenate found could then be explained as contamination of the carrier material. Deoxyadenosine monoarsenate (dAMAs), the structural arsenic analogue of the DNA building block dAMP, has a half-life of 40 minutes in water at neutral pH. In a study of the kinetic consequences of substituting the phosphorus diesters in the DNA strand with arsenic diesters, a working group from the University of Missouri and the University of Cairo found that the latter has an estimated half-life to decomposition through spontaneous hydrolysis at a water temperature of 25 ° C of only 0.06 seconds compared to the half-life of phosphorus diesters of about 30,000,000 (30 million) years.

They believed that a living being with such unstable genetic material would be viable.

In May 2011, Science published eight replies to the original publication, in which the researchers were accused of poor cleaning of the samples and inexplicable discrepancies in the preparation of the results. In a further comment, the research team explained the methodology of their research to the original publication, responded to criticism and reiterated that the basic assumption of the substitution of phosphate by arsenic would be maintained. From now on, samples of the bacterium are also available to other research groups.

In June 2012, a group led by Rosemary Redfield published the results of mass spectrometric studies on arXiv , in which they were able to detect free arsenate in DNA preparations made from GFAJ-1, but no arsenic integrated into the DNA. They concluded that although the bacteria tolerate arsenic, they could differentiate between these two ions so well even with very small amounts of available phosphate that the biochemistry of their nucleic acids corresponds to that of other bacteria.

literature

• Felisa Wolfe-Simon, Jodi Switzer Blum, Thomas R. Kulp, Gwyneth W. Gordon, Shelley E. Hoeft, Jennifer Pett-Ridge, John F. Stolz, Samuel M. Webb, Peter K. Weber, Paul CW Davies, Ariel D Anbar, Ronald S. Oremland: A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus. In: Science , 2010; doi: 10.1126 / science.1197258 .
• Marshall L. Reaves, Sunita S. Sinha, Joshua D. Rabinowitz, Leonid Kruglyak, Rosemary J. Redfield: Absence of detectable arsenate in DNA from arsenate-grown GFAJ-1 cells. In: Science . Volume 337, number 6093, July 2012, pp. 470-473, doi : 10.1126 / science.1219861 , PMID 22773140 , PMC 3845625 (free full text).

Web links

 Wikinews: NASA discovers new life form: bacterial strain GFAJ-1  - message

• Press conference to announce NASA's discovery of GFAJ-1

Individual evidence

1. ^ ^{A b} Marshall Louis Reaves, Sunita Sinha, Joshua D. Rabinowitz, Leonid Kruglyak, Rosemary J. Redfield: Absence of detectable arsenate in DNA from arsenate-grown GFAJ-1 cells . 2012, arxiv : 1201.6643 (English).
2. ^ Taz: NASA research report rejected , July 9, 2012
3. ↑ Deutschlandfunk, Forschung Aktuell: Picky Microbes: How Organisms Differentiate Between Phosphorus and Arsenic , October 11, 2012
4. ↑ NASA-Funded Research Discovers Life Built With Toxic Chemical on the homepage of the National Aeronautics and Space Administration (English)
5. ↑ N. Weber and A. Bojanowski: Nasa discovers spectacular life form. at Spiegel Online from December 2, 2010
6. ↑ Deutschlandfunk: Forschungs aktuell - Critique of Arsenic Bacteria Find , broadcast on December 3, 2010 (also as audio on demand , running time 2:58 min)
7. ↑ Astrobiology online from December 2, 2010 [1]
8. ↑ Nature News of December 2, 2010 [2]
9. ↑ ^{a b} Alex Bradley et al. : Archive link ( Memento from December 8, 2010 in the Internet Archive ), US Science Blogs from December 5, 2010
10. ↑ Rose Redfield in her own research blog from December 4, 2010 [3]
11. ↑ Rosario Lagunas, David Pestana, Jose C. Diez-Masa: Arsenic mononucleotides. Separation by high-performance liquid chromatography and identification with myokinase and adenylate deaminase . In: Biochemistry . 23, No. 5, 1984, pp. 955-960. doi : 10.1021 / bi00300a024 . PMID 6324859 .
12. ↑ Mostafa I. Fekry, Peter A. Tipton, Kent S. Gates: Kinetic Consequences of Replacing the Internucleotide Phosphorus Atoms in DNA with Arsenic. ACS Chemical Biology 2011, 6 (2); Pp. 127-130. doi: 10.1021 / cb2000023
13. ↑ Bruce Alberts: Editor's Note , Science, May 27, 2011
14. ↑ Felisa Wolfe-Simon, Jodi Switzer Blum, et al .: Response to Comments on “A Bacterium That Can Grow Using Arsenic Instead of Phosphorus” (PDF file; 141 kB) , Science, May 27, 2011