Negative and positive regulation of the non-osmoregulated ompS1 porin genein Salmonella typhi: a novel regulatory mechanism that involves OmpR

The Salmonella typhi ompS1 gene codes for an outer membrane protein of the OmpC/OmpF porin family, It is expressed at very low levels, relative to the major porins, However, deletion analysis of the 5' regulatory region showe d that the gradual removal of nucleotides -310 to -88, upstream from the P1 major transcriptional start-point, resulted in a stepwise increase in expr ession, reaching levels 10-fold above those for the ompC major porin gene. Hence, this 222 bp segment contains cis-acting regulatory elements involved in negative control. Primer extension analysis revealed the presence of th ree promoters: P1 activity was OmpR dependent; P2 was expressed at a fewer level in the absence of OmpR; and P3 had a minor constitutive activity, Omp R bound preferentially to box II, an 18 bp F1/C1 canonical site, the remova l (-88 to -66) of which resulted in a decrease in expression thus supportin g its role in positive control, Expression of ompS1 was not induced by a se t of stress conditions, including a shift in osmolarity, nor was the IHF re gulator involved in negative control, An ompS1 homologue was found in E, co li K-12, which contains a nonsense codon and a shift in the reading frame, whereas Salmonella typhimurium contains an open reading frame in this regio n. Thus, S. typhi ompS1 provides novel features in OmpR regulation.