FIG 3.

Labeling strategy for distinguishing the pathway for l-methionine biosynthesis from a mixture of labeled and unlabeled DMSP. Both the direct capture and reassembly pathways result in l-methionine that is either unenriched (mass shift of 0) or enriched with both ¹³C and ³⁴S (mass shift of 3). However, the reassembly pathway also produces l-methionine that is enriched in either ¹³C or ³⁴S, i.e., singly labeled (mass shifts of 1 and 2, respectively). Filled circles indicate atoms that are enriched with either ¹³C or ³⁴S. Open circles indicate atoms with the natural abundance of isotopes. Abbreviations: Glc, glucose; 3PG, 3-phosphoglycerate; Ser, l-serine; THF, tetrahydrofolate.