Triplicate chemostat cultivations were carried out with L. lactis subsp. lactis IL1403 at specific growth rates 0.1 and 0.5 h-1 on a chemically defined medium, where both, glucose and lysin (Lys) were jointly limiting the growth. After reaching the steady physiological state, medium was switched to heavy Lys (13C615N2-Lys) containing medium and incorporation of heavy amino acid into biomass was observed over the time course. Samples were collected before the medium switch (0 h) and 0.5; 1.5; 5; and 10 h after the medium switch. Cells were lysed in SDS lysis buffer and digested with trypsin according to Filter Aided Sample Preparation protocol (FASP). LC-MS/MS analysis were performed on Agilent 1200 series nanoflow system connected to a LTQ Orbitrap mass-spectrometer equipped with a nanoelectrospray ion source. Raw data files were analyzed with the MaxQuant software package (version 1.2.7.4). Generated peak lists were searched using the Andromeda search engine (built into MaxQuant) against L. lactis database (downloaded 28.06.2012 from http://www.genome.jp/kegg/genes.html). MaxQuant searches were performed with full tryptic specificity, a maximum of two missed cleavages and a mass tolerance of 0.5 Da for fragment ions. Carbamidomethylation of cysteine was set as a fixed modification and methionine oxidation and protein N-terminal acetylation were set as variable modification. The required false discovery rate (FDR) was set to 1% both for peptide and protein levels and the minimum required peptide length was set to six amino acids. In addition, "Match between runs" option with a time window of 1.5 min was allowed.