A short time ago, a study describing the discovery of a bacterium that could apparently utilize arsenate in place of phosphate was published in Science. This report has since generated a lot of discussion and debate in the blog-o-sphere, most of it rather uncomplementary. I count myself among those who are skeptical of the more daring of the authors’ claims.
Something I found curious was the gel in Fig. 2A, an analysis of nucleic acids isolated from bacteria grown in arsenate-rich (middle lane) or phosphate-rich (right lane) media:
Something is missing in the middle lane – the ribosomal RNA that is so abundant in the right-hand lane. Possibly related to this curious result, the authors make this statement that is hidden away in the paper:
“Since these cells were harvested in stationary phase (11), the fraction of P associated with RNA is likely small (14).”
These authors seem to be implying that cells grown in arsenate have little RNA, far less than DNA. This seems very unlikely to me. Even slowly-growing cells should have as much or more ribosomal RNA as DNA. Indeed, in contrast to what the authors are claiming here, reference 14 mentions that total RNA levels in non-growing bacterial cells do not change very much.
Reference 11 is the methods, and reference 14 is: J. Mandelstam, Bactierol Rev. 24, 289 (1960). The authors of the Science paper seem to have mixed up a discussion of RNA stability with a consideration of overall RNA quantities. On p.297, Mandelstam states in this regard:
“In experiments with a leucine auxotroph (26) cells were labeled with P32-orthophosphate and then starved of leucine in an unlabeled medium. About 20 per cent of the p32 of the RNA was lost in 90 min and at least half of this was recovered from the incubation medium. There must have been a good deal of resynthesis of RNA from unlabeled phosphorus, because the total RNA decreased only 0 to 12 per cent, which is far less than the decrease in label.”
Obviously, the study Mandelstam cites does not claim that total RNA levels decrease dramatically (let alone to the extent that would have to be the case in the arsenate-grown cells) in non-growing cells.
I don’t have any ready explanation for this figure, but I am not comfortable with the authors’ explanations. There would seem to be more to these cells than meets the eye.