The latest I’ve come across on the subject of the mooted arsenic-using Mono Lake bacteria is Carl Zimmer’s post at The Loom. As he concludes in his overview of the whole affair, nearly two years old, the bacteria in question only tolerate arsenic, not make use of it; but what an “only”!
Imagine what it’s like for a microbe in Mono Lake, or in the lab of a particularly sadistic scientist. You’re drowning in arsenate, and in order to stay alive, to keep growing, you need to grab the precious few phosphate molecules drifting by.
Dan Tawfik, an expert on protein function at the Weizmann Institute in Israel, and his colleagues have uncovered some of GFAJ-1′s secrets to survival. GFAJ-1 and other bacteria absorb phosphate through their outer membrane, into a sandwiched layer of fluid called the periplasm. Once there, the phosphate is grabbed by so-called phosphate binding proteins, which then deliver the phosphate to the interior of the microbe. Tawfik and his colleagues examined these proteins in unprecedented detail to see how they work.
The scientists offered the proteins a mixture of arsenate and phosphorus. Even when they raised the ratio to 500 molecules of arsenate to every phosphate molecule, the proteins still managed to pluck out phosphate over half the time. The scientists then examined the proteins to figure out how they make such fine discriminations. When the proteins encounter a molecule of phosphate, they enfold it in a tight pocket, which ties down the phosphate with 12 different hydrogen bonds. When arsenate falls into that pocket, it doesn’t quite fit in, and the bond between one of the oxygen atoms in the arsenate and one of the hydrogen atoms in the protein gets twisted. It gets pushed to such an uncomfortable angle that the arsenate drops out.
This finding suggests that ordinary microbes are well-adapted to picking out phosphates when they’re scarce, using their fussy phosphate binding proteins to reject abundant arsenate. GFAJ-1 is stuck in a place where phosphate is always scarce and arsenate is always dangerously copious. Tawfik and his colleagues found that one form of their phosphate binding proteins is spectacularly fussy, preferring phosphates by a factor of 4,500. What’s more, GFAJ-1 produces many copies of this super-fussy protein. As a result, GFAJ-1 can thrive in Mono Lake. In fact, it can handle arsenate-to-phosphate ratios up to 3,000 times higher than found in the lake.