Figure 7.

SILAC labeling of yeast to recognize true peptide signals. A yeast strain that is deficient for lysine biosynthesis is grown in the presence of normal lysine or lysine with substituted ¹³C and ¹⁵N, leading to a mass difference of 8 Da. Yeast cells are mixed in equal proportions, lysed, digested by endopeptidase LysC and analyzed by mass spectrometry. In the example mass spectrum, each true peptide signal is represented by a pair, spaced by 8 Da (blue arrows; mass difference appear different because peptide can have different charge states). Peaks marked by red stars are unlikely to be yeast peptides because they have no SILAC partner.