EFFECT OF MUTATIONS IN THE -10-REGION OF THE PHOE PROMOTER IN ESCHERICHIA-COLI ON REGULATION OF GENE-EXPRESSION

The phoE promoter region in Escherichia coli contains a - 10 region, t ypical of sigma(70)-dependent promoters and, instead of a normal - 35 region, a so-called pho box, to which the transcriptional activator ph ospho-PhoB binds under low phosphate conditions. A second pho box is p resent upstream of the first one and is required for full expression o f phoE during phosphate starvation. To determine whether the lack of e xpression under high phosphate conditions is due solely to the absence of a genuine - 35 box, the - 10 region was further optimized towards the consensus - 10 sequence and promoter activity was measured using a lkaline phosphatase as a reporter. The mutations resulted in a drastic increment in the basal level of expression under high phosphate condi tions, indicating that the deviations from consensus in the - 10 regio n also play a role in determining the poor expression of the wild-type promoter under these conditions. The expression under high phosphate conditions was partly dependent on the presence of the phoB gene, show ing that a small amount of active PhoB must be present under these cir cumstances. During phosphate starvation, the activity of the mutant pr omoters was further induced. The upstream pho box was not required for full expression from the mutant promoters under these conditions. App arently, the wild-type phoE promoter is carefully balanced by deviatio ns from the optimal Prib-now box sequence that reduce expression under high phosphate conditions and by the presence of several copies of th e pho box, which enhance expression under phosphate starvation.