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.
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