The function of micF RNA. micF RNA is a major factor in the thermal regulation of OmpF protein in Escherichia coli

The role of chromosomally derived micF RNA as a repressor of outer membrane protein OmpF of Escherichia coli was examined for various growth conditions. Levels of micF RNA as determined by Northern analyses are found to increase in response to cell growth at high temperature, in high osmolarity or in the presence of ethanol. After a switch to higher growth temperature, the levels of ompF mRNA and of newly synthesized OmpF decrease with time in E. coli strain, MC4100 but these decreases are not observed in isogenic micF deletion strain, SM3001. In addition, while levels of ompF mRNA are substantially reduced in both strains in response to high osmolarity or ethanol at 24 degrees C, the reduced levels in the parental strain are still 4-5-fold lower compared with the micF deletion strain. These findings indicate that chromosomally derived micF RNA plays a major role in the thermal regulation of OmpF and represses OmpF synthesis in response to several environmental signals by decreasing the levels of ompF mRNA. Analyses of the effect of a multicopy micF plasmid on the levels of OmpF and ompF mRNA after an increase in temperature indicated that multicopies of micF RNA markedly inhibited OmpF synthesis but did not accentuate ompF mRNA decrease. These data suggest that multicopy micF inhibits OmpF synthesis primarily through translational inactivation of ompF mRNA and that a limiting factor in addition to micF RNA is necessary to destabilize ompF mRNA.     

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 increases with increased osmolarity when cells are growing in neutral and alkaline media, whereas the level of OmpF decreases at high osmolarity, and that (ii) the two-component system composed of OmpR (regulator) and EnvZ (sensor) regulates porin expression. In this study, we found that OmpC was expressed at low osmolarity in medium of pH below 6 and that the expression was repressed when medium osmolarity was increased. In contrast, the expression of ompF at acidic pH was essentially the same as that at alkaline pH. Neither OmpC nor OmpF was detectable in an ompR mutant at both acid and alkaline pH values. However, OmpC and OmpF were well expressed at acid pH in a mutant envZ strain, and their expression was regulated by medium osmolarity. Thus, it appears that E. coli has a different mechanism for porin expression 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 diminution was observed when ompC and ompF were deleted, suggesting that both OmpF and OmpC are required for optimal growth under hypoosmosis at acid pH.     

Expression of micF involved in porin synthesis in Escherichia coli: two distinct cis-acting elements respectively regulate micF expression positively and negatively

micF RNA, whose sequence is highly complementary to a 5'-portion of ompF mRNA, has been implicated in the osmoregulation and thermoregulation of the ompF porin gene in Escherichia coli. To define and characterize cis-acting regulatory regions upstream of the micF promoter, a series of deletions of the micF promoter fused to the lacZ gene were constructed. Two distinct regions, which function differently, were identified as cis-acting regulatory elements, namely, one responsible for OmpR-dependent activation and the other for OmpR-independent repression of micF expression. The former contains the OmpR-binding site, which simultaneously regulates both the genes, micF and ompC, in response to the medium osmolarity. The latter may be involved in an unknown regulatory process of micF expression.