THE RELATION BETWEEN PPGPP AND THE PHO REGULON IN ESCHERICHIA-COLI

Starving of Escherichia coli cells for inorganic orthophosphate (Pi) r esults in the accumulation of spoT-dependent ppGpp and induces the exp ression 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 constitu tive mutants. The transcription of phoA and of pstS is not affected in the ppGpp(0) strain, and therefore this impairment is due to a post-t ranscriptional defect. Conversely, overexpression of ppGpp inhibits tr anscription of these two PHO regulon genes. In phoB mutants, the accum ulation of ppGpp during Pi starvation is diminished, suggesting that P hoB or one of the PHO products is involved in the control of ppGpp acc umulation. We propose that the presence of ppGpp in the cell, but not its accumulation as a result of the starvation stress, is important fo r the expression of the PHO genes.