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Systems biology based on high quality absolute quantification data, which are mandatory for the simulation of biological processes, successively becomes important for life sciences. We provide protein concentrations on the level of molecules per cell for more than 700 cytosolic proteins of the Gram-positive model bacterium Bacillus subtilis during adaptation to changing growth conditions. As glucose starvation and heat stress are typical challenges in B. subtilis' natural environment and induce both, specific and general stress and starvation proteins, these conditions were selected as models for starvation and stress responses. Analyzing samples from numerous time points along the bacterial growth curve yielded reliable and physiologically relevant data suitable for modeling of cellular regulation under altered growth conditions. The analysis of the adaptational processes based on protein molecules per cell revealed stress-specific modulation of general adaptive responses in terms of protein amount and proteome composition.Furthermore, analysis of protein repartition during glucose starvation showed that biomass seems to be redistributed from proteins involved in amino acid biosynthesis to enzymes of the central carbon metabolism. In contrast, during heat stress most resources of the cell, namely those from amino acid synthetic pathways, are used to increase the amount of chaperones and proteases. Analysis of dynamical aspects of protein synthesis during heat stress adaptation revealed, that these proteins make up almost 30% of the protein mass accumulated during early phases of this stress. Recently technical approaches in systems biology have become more and more important for the life science community. Successful modeling of biological pathways as part of these approaches strongly depends on quantitative, highquality, and validated data sets (1). Proteins are an important part of these attempts to uncover the systemic properties of biological systems as they represent the central players in the complex cellular metabolic and adaptational network (2).Although relative protein quantification methods allow for comparison of protein abundances in samples and to characterize the proteome dynamics in cellular systems, these data are not sufficient for mathematical modeling in systems biology. Furthermore, the availability of protein concentrations at the proteome level can provide new insights in what is going on in the cell upon stress, and thus enable us to better understand how cells adapt to changing conditions. Knowing intracellular protein concentrations is essential in order to obtain a real mass balance leading to evaluation of the costs of running an active metabolic pathway or expressing enzymes for stress responses. In order to provide suitable proteomic data for systems biology, techniques for global absolute quantification of proteins recently emerged. These approaches make use of quantitative Western blotting (3), mass spectrometry (4, 5), or merge traditional two-dimensional polyacrylamide gel elec...

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