Transcription of rpoH, encoding the Escherichia coli heat‐shock regulator σ32, is negatively controlled by the cAMP‐CRP/CytR nucleoprotein complex

Abstract

In Escherichia coli, the rpoH gene encoding the essential heat-shock regulator σ³², is expressed in a complex manner. Transcription occurs from four promoters (P1, P3, P4 and P5) and is modulated by several factors including (i) two σ factors (σ⁷⁰ and σ^E); (ii) the global regulator CRP; and (iii) the DnaA protein. Here, a further dissection of the rpoH regulatory region has revealed that an additional transcription control exists that appears to link rpoH expression to nucleoside metabolism. The cAMP–CRP complex and the CytR anti-activator bind co-operatively to the promoter region forming a repression complex that overlaps the σ^E-dependent P3 promoter and the σ⁷⁰-dependent P4 and P5 promoters. During steady-state growth conditions with glycerol as the carbon and energy source, transcription from P3, P4 and P5 is reduced ≈threefold by CytR, whereas transcription from the upstream promoter, P1, appears to be unaffected. Furthermore, in strains that slightly overproduce CytR, transcription from P3, P4 and P5 is reduced even further (≈10-fold), and repression can be fully neutralized by the addition of the inducer cytidine to the growth medium. In the induced state, P4 is the strongest promoter and, together with P3 and P5, it is responsible for most rpoH transcription (65–70%). At present, CytR has been shown to ‘fine tune’ transcription of two genes (rpoH and ppiA) that are connected with protein-folding activities. These findings suggest that additional assistance in protein folding is required under conditions in which CytR is induced (i.e. in the presence of nucleosides).