The relation between ppGpp and the PHO regulon in Escherichia coli

Starving of Escherichia coli cells for inorganic orthophosphate (Pi) results in the accumulation of spoT-dependent ppGpp and induces the expression of genes of the PHO regulon. In a delta relA delta spoT strain that is unable to accumulate ppGpp the expression of two genes (phoA and pstS) that belong to the PHO regulon is impaired, even in constitutive mutants. The transcription of phoA and of pstS is not affected in the ppGpp0 strain, and therefore this impairment is due to a post-transcriptional defect. Conversely, overexpression of ppGpp inhibits transcription of these two PHO regulon genes. In phoB mutants, the accumulation of ppGpp during Pi starvation is diminished, suggesting that PhoB or one of the PHO products is involved in the control of ppGpp accumulation. We propose that the presence of ppGpp in the cell, but not its accumulation as a result of the starvation stress, is important for the expression of the PHO genes.