The ompB operon partially determines differential expression of OmpC in Salmonella typhi and Escherichia coli

Expression of the Escherichia coli OmpC and OmpF outer membrane proteins is regulated by the osmolarity of the culture media. In contrast, expression of OmpC in Salmonella typhi is not influenced by osmolarity, while OmpF is regulated as in E. coli. To better understand the lack of osmoregulation of OmpC expression in S. typhi, we compared the expression of the ompC gene i n S. typhi and E. coli, using ompC-lacZ fusions and outer membrane protein (OMP) electrophoretic profiles. S. typhi ompC expression levels in S. typhi were similar at low and high osmolarity along the growth curve, whereas os moregulation off. coli ompC in E. coli was observed during the exponential phase. Both genes were highly expressed at high and low osmolarity when pre sent in S. typhi, while expression of both was regulated by osmolarity in E . coli. Complementation experiments with either the S. typhi or E. coli omp B operon in an S. typhi Delta ompB strain carrying the ompC-lacZ fusions sh owed that both S. typhi and E. coli ompC were not regulated by osmolarity w hen they were under the control of S. typhi ompB. Interestingly, in the sam e strain, both genes were osmoregulated under E. coli ompB. Surprisingly, i n E. coli Delta ompB, they were both osmoregulated under S. typhi or E. col i ompB. Thus. the lack of osmoregulation of OmpC expression in S. typhi is determined in part by the ompR operon, as well as by other unknown trans-ac ting elements present in S. typhi.