Expression of outer membrane proteins in Escherichia coli growing at acid pH

It is generally accepted for Escherichia coli that (i) the level of OmpC in creases with increased osmolarity when cells are growing in neutral and alk aline media, whereas the level of OmpF decreases at high osmolarity, and th at (ii) the two-component system composed of OmpR (regulator) and EnvZ (sen sor) regulates porin expression. In this study, we found that OmpC was expr essed at low osmolarity in medium of pH below 6 and that the expression was repressed when medium osmolarity was increased. In contrast, the expressio n of ompF at acidic pH was essentially the same as that at alkaline pH. Nei ther OmpC nor OmpF was detectable in an ompR mutant at both acid and alkali ne pH values. However, OmpC and OmpF were well expressed at acid pH in a mu tant envZ strain, and their expression was regulated by medium osmolarity, Thus, it appears that E. coli has a different mechanism for porin expressio n at acid pH. A mutant deficient in ompR grew slower than its parent strain in low-osmolarity medium at acid pH (below 5.5). The same growth diminutio n was observed when ompC and ompF were deleted, suggesting that both OmpF a nd OmpC are required for optimal growth under hypoosmosis at acid pH.