Researchers that claimed that the GFAJ-1 bacterium had a preference for arsenic instead of phosphorous in its DNA have been somewhat refuted. A new study shows that the GFAJ-1 microbe actually goes to extreme lengths to grab any traces of phosphorous it can find.
This clears up one of the lingering questions, from a controversial study published in the journal Science, that claimed that the GFAJ-1 microbe could thrive in the high-arsenic conditions of Mono Lake in California without metabolizing phosphorous, an element that’s essential to all forms of known life. The scientists published their findings in the journal Nature.
The scientists have discovered how the bacteria discriminate between nearly identical molecules of phosphate and arsenate. They looked at five types of phosphate-binding proteins, which help bind phosphate in a molecular pathway that brings it into cells, from four species of bacteria. Two of the species were sensitive to arsenate, while two others were resistant to it.
The threshold for when “discrimination” broke down was when 50% of the proteins ended up bound to arsenate, indicating that the ability to discriminate had been overwhelmed. Even in solutions containing 500 times more arsenate than phosphate, all five proteins were still able to preferentially bind phosphate. One protein that was derived from GFAJ-1 could do so at arsenate excesses of up to 4,500-fold over phosphate.
The detailed structure of Pseudomonas fluorescens, the phosphate binding protein, showed that the arsenate molecule, which is slightly larger than phosphate, distorts and weakens bonds around a hydrogen atom that forms a bridge to the protein.
This doesn’t mean that the arsenate doesn’t get into the bacteria, it just shows that the bacterium has evolved to extract phosphate under almost any circumstances.