Alkaline phosphatase as a reporter of σS levels and rpoS polymorphisms in different E. coli strains
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σS is responsible for the transcriptional regulation of genes related to protection against stresses and bacterial survival and it accumulates in the cell under conditions of stress, such as nutrient limitation. An increase in the levels of σS causes a reduction in the expression of genes that are transcribed by RNA polymerase associated with the principal sigma factor, σ70. phoA, that encodes alkaline phosphatase (AP) is expressed under phosphate shortage conditions, and is also repressed by σS. Here we show that in a Pi-limited chemostat, accumulation of rpoS mutations is proportional to the intrinsic level of σS in the cells. Acquisition of mutations in rpoS relieves repression of the PHO genes. We also devised a non-destructive method based on the rpoS effect on AP that differentiates between rpoS+ and rpoS mutants, as well as between high and low-σS producers. Using this method, we provide evidence that σS contributes to the repression of AP under conditions of Pi excess and that AP variation among different strains is at least partly due to intrinsic variation in σS levels. Consequently, a simple and non-destructive AP assay can be employed to differentiate between strains expressing different levels of σS on agar plates.
KeywordsrpoS Sigma factor Alkaline phosphatase PHO regulon
This work was supported by Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP- Brazil) and the Endeavour Research Fellowships (Australia).
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