I've got lots of GFAJ-1 cells in the fridge, so I think I'll make some DNA preps from them to send to my colleagues at Princeton for the initial mass-spec measurement of baseline levels of arsenic. They've done the initial controls, determining that their setup's detection limit is about 1-^-7 - 10^-8 M arsenate, equivalent to replacement of about 0.01% of the phosphates in the DNA backbone with arsenate. This is about 300-fold lower than the 4% replacement claimed by Wolfe-Simon et al.
The cells were grown in AML60 medium with either limiting or ample phosphate - I don't think it matters which. I'll do one DNA prep with the limiting-phosphate cells. As a control for carry-over arsenate contamination, I'll add sodium arsenate to the other culture (40 mM) and let them steep in it for a little while, then wash away the arsenate and do another DNA prep. I'll also take some purified DNA, soak it in 40 mM arsenate for a little while, and then repurify it. If my DNA purification methods are good, none of the DNAs should have detectable arsenic.
The Newton Medal is (bit) late
11 hours ago in Doc Madhattan