"Tawfik says that he was shocked by how good the proteins were at discriminating between the essential phosphate and the deadly arsenate. This does not mean that arsenate does not get into the bacteria, he points out. "It just shows that this bacterium has evolved to extract phosphate under almost all circumstances." The exceedingly high preference for phosphorous found in the key proteins in that species represent "just the last nail in the coffin" of the hypothesis that GFAJ-1 uses arsenic in its DNA, says Tawfik."
NASA's Big Arsenic-Based Life Claim Was Wrong, earlier post