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

Using a set of gene fusions generated with TnphoA, we previously ident ified the phmA locus, whose expression is modulated as a function of e xternal pH (pHo). The phmA=phoA fusion was cloned and sequenced and th e phmA locus was identified with the nmpC gene. This gene lies within the defective lambdoid prophage qsr' and NmpC is an outer membrane pro tein which functions as a porin. We demonstrated that nmpC is sensitiv e to catabolite repression and dependent on the CRP-cAMP complex. Howe ver, cAMP is not a signal for the pHo-dependent expression of nmpC. By generating step deletions in the sequence 5' to the nmpC coding regio n, we identified a DNA region in position -345 to -127 which is involv ed in nmpC repression, mainly during growth at acid pHo. Four regions with strong homologies and a very well-conserved organization of the f unctional sequence were found in the nmpC and ompF promoters. We propo se that the negative regulation of nmpC during growth at low pHo might involve DNA looping of the nmpC promoter.