A two-dimensional (2-D) gel electrophoresis study of strain 168 identified 20 proteins that are strongly induced in response to phosphate starvation. The induction of nine of these phosphate-starvation-induced (Psi) proteins was dependent on a functional PhoR protein. PhoR is the histidine sensor-kinase component of a phosphate-concentration-sensing two-component regulatory system which, together with its partner response regulator PhoP, controls the expression of genes in the Pho regulon. Genes encoding PhoR-dependent Psi proteins are therefore likely to be members of the Pho regulon. SpoOA ~ P, the response regulator of the signal transduction pathway required for the induction of sporulation, has previously been shown to negatively affect the induction of the Pho regulon by repressing the operon. The induction pattern of some PhoR-dependent Psi proteins was altered in a mutant such that their synthesis continued for longer than was found with the wild-type. The most abundant Psi protein, Psi1-3, was characterized by N-terminal sequencing of internal peptide fragments and shown to have a high similarity to an protein which is involved in phosphate uptake during phosphate starvation.


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