The behavior of citrulline during growth. Changes in metabolite concentration were measured throughout the growth curve of H. salinarum NRC-1. Some metabolites showed coordinated behavior with the gene expression of enzymes that catalyze their synthesis or degradation. (A) The metabolic pathways involved in citrulline degradation and biosynthesis in H. salinarum NRC-1. Gene names encoding enzymes catalyzing the illustrated metabolic reactions are shown near the arrows. (B) Abundance profile for citrulline across the three growth phases, early growth, exponential growth and stationary phase. (C) Gene expression profiles for genes involved in arginine fermentation, vng6315G, vng6316G and vng6317G. Note the similar patterns in the changes of abundance of these three genes and the abundance of citrulline. (D) Gene expression profiles for vng2436G and vng2437G. The enzymes encoded by these genes catalyze the innerconversion of cirtulline to arginine. Note the differential regulation of these genes with those highlighted in panel C. While both pathways are active during stationary phase, the abundance of citrulline is nevertheless best predicted by the activity of vng6315G, vng6316G and vng6317G.