FIG 1.

Examples of MRM spectra for adenine and guanine for three samples: unenriched DNA from a Gluconacetobacter sp. culture (a), ¹³C-enriched DNA from Gluconacetobacter sp. grown on pure [¹³C]glucose (b), and ¹³C-enriched DNA recovered from a high density fraction following the ultracentrifugation of DNA extracted from a soil incubation with ¹³C-cellulose (c). The differences in retention times between Fig. 1 and Fig. S2 in the supplemental material are due to the use of different chromatography columns and mobile phases, as described in Materials and Methods.