The intracellular nucleotide pool of Escherichia coli W3110 reproducibly changes from conditions of growth in phosphate excess to phosphate starvation, with at least two nucleotides appearing under starvation conditions and two nucleotides appearing only under excess phosphate conditions. Strains bearing a deletion of the phoA gene show the same pattern, indicating that dephosphorylation by alkaline phosphatase is not responsible for the changes. Strains with mutations in the phoU gene, which result in constitutive expression of the pho regulon, show the nucleotide pattern of phosphate-starved cells even during phosphate excess growth. These changes in nucleotides are therefore due to phoU mutation but not to alkaline phosphatase constitutivity. In fact, a phoR (phoR68) mutant strain has the patterns of the wild type in spite of being constitutive for alkaline phosphatase. That these nucleotides might be specific signals for pho regulon expression was supported by the fact that the two nucleotides appearing under phosphate starvation induced the synthesis of alkaline phosphatase in repressed permeabilized wild-type cells under conditions of phosphate excess
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