Expression of the nmpC gene of Escherichia coli K‐12 is modulated by external pH. Identification of cis‐acting regulatory sequences involved in this regulation

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Abstract
Using a set of gene fusions generated with TnphoA, we previously identified the phmA locus, whose expression is modulated as a function of external pH (pHo). The phmA::phoA fusion was cloned and sequenced and the phmA locus was identified with the nmpC gene. This gene lies within the defective lambdoid prophage qsr' and NmpC is an outer membrane protein which functions as a porin. We demonstrated that nmpC is sensitive to catabolite repression and dependent on the CRP-cAMP complex. However, cAMP is not a signal for the pHo-dependent expression of nmpC. By generating step deletions in the sequence 5' to the nmpC coding region, we identified a DNA region in position -345 to -127 which is involved in nmpC repression, mainly during growth at acid pHo. Four regions with strong homologies and a very well-conserved organization of the functional sequence were found in the nmpC and ompF promoters. We propose that the negative regulation of nmpC during growth at low pHo might involve DNA looping of the nmpC promoter.