Andreas Haldimann

Are the Multiple Signal Transduction Pathways of the Pho Regulon Due to Cross Talk or Cross Regulation

The phosphate (Pho) regulon includes a large number of coregulated genes whose expression is controlled by the environmental (extracellular) inorganic phosphate (Pi) level. Altogether 11 Pho regulon promoters for a total of 38 different genes have now been characterized in E. coli or closely related bacteria (Fig. 15.1). The expression of most of them has been shown to require the transcription factor PhoB that upon phosphorylation activates transcription by binding to Pho Box sequences within the respective promoter region of each of these genes or operons. The E. coli Pho regulon currently comprises 31 genes that are arranged in eight unlinked transcriptional units. At least 13 of these genes are also believed to exist in Salmonella typhimurium, although 16 others are known to be absent. In addition, S. typhimurium has seven Pho regulon genes that are absent in E. coli. Most of the corresponding gene products have roles in the use of various phosphorus (P) compounds as sole P sources for growth, while the roles of others are unknown.1