A basic challenge in cell biology is to understand how interconnected metabolic pathways are regulated to provide the adequate cellular outcome when changing levels of metabolites and enzyme expression. In Escherichia coli, the arginine and pyrimidine biosynthetic pathways are connected through a common metabolite provided by a single enzyme. The different elements of the arginine biosynthetic system of Escherichia coli, including the connection with pyrimidine biosynthesis, and the principal regulatory mechanisms operating at genetic and enzymatic levels were integrated in a mathematical model using a molecular kinetic approach combined with a modular description of the system. The model was then used to simulate a set of perturbed conditions as follows: genetic derepression, feedback resistance of the first enzymatic step, and low constitutive synthesis of the intermediate carbamyl phosphate. In all cases, an excellent quantitative agreement between simulations and experimental results was found. The model was used to gain further insight into the function of the system, including the synergy between the different regulations. The outcome of combinations of perturbations on cellular arginine concentration was predicted accurately, establishing the model as a powerful tool for the design of arginine-overproducing strains.One of the major aims of systems biology is to achieve a global understanding of metabolism by developing models able to describe and predict molecular and cellular functions at the level of whole metabolic pathways (1). Ultimately, the objective is to obtain comprehensive computer models of an entire living cell and all its functions. To attain such a goal, the International Escherichia coli Alliance was launched in 2002 to consolidate global E. coli modeling efforts. As a part of this global project, we developed a reliable mathematical model for arginine biosynthesis, based on the existing detailed knowledge of the principal regulatory mechanisms operating at both the genetic and the enzymatic levels of arginine metabolism (for reviews see Refs. 2, 3) as well as on new experimental determinations. A molecular kinetic model will be used that is appropriate for the description of highly regulated systems, in particular to establish a hierarchy of the different regulatory mechanisms at work in the system (4). This model can then be used to predict arginine production in strains containing different modifications of the regulatory circuits.Arginine constitutes about 5% of the total proteins of E. coli.