The effect of IHF on σ^S selectivity of the phoA and pst promoters of Escherichia coli

The pst operon, a member of the PHO regulon of Escherichia coli, encodes a high-affinity phosphate transport system whose expression is induced when the cells enter a phase of phosphate starvation. The expression of pst is stimulated by the integration host factor (IHF). Transcription of the PHO regulon genes is initiated by the RNA polymerase complexed with σ^D (Eσ^D). Owing to a cytosine residue at position -13 of the pst promoter its transcription can also be initiated by Eσ^S. Here, we show that inactivation of IHF in vivo abolishes the σ^S- dependent transcription initiation of the pst operon, indicating that both -13C residue and IHF are required to confer on pst the ability to be transcribed by Eσ^S. Introduction of a -13C residue in the promoter region of phoA, another PHO regulon gene that is not directly affected by IHF, did not affect its exclusive transcription initiation by Eσ^D.