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 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.     

The use of RNAs complementary to specific mRNAs to regulate the expression of individual bacterial genes

A naturally occurring small RNA molecule ( micF RNA), complementary to the region encompassing the Shine-Dalgarno sequence and initiation codon of the ompF mRNA, is known to block the expression of that mRNA in E. coli. We have constructed a plasmid that produces a complementary RNA to the E. coli lpp mRNA (mic[Ipp] RNA). Induction of the mic(Ipp) gene efficiently blocked lipoprotein production and reduced the amount of lpp mRNA. Two mic(ompC) genes were similarly engineered and their expression was found to inhibit drastically production of OmpC. Analysis of several types of mic(ompA) genes suggests that micRNAs complementary to regions of the mRNA likely to come in contact with ribosomes were most effective. The novel capabilities of this artificial mic system provide great potential for application in both procaryotic and eucaryotic cells.     

Secretion into the culture medium of a foreign gene product from Escherichia coli: use of the ompF gene for secretion of human beta-endorphin.

The ompF gene codes for a major outer membrane protein of Escherichia coli. A plasmid was constructed in which the structural gene for human beta-endorphin is preceded by the upstream region of the ompF gene consisting of the promoter region and the coding regions for the signal peptide and the N terminus of the OmpF protein. When the plasmid was introduced into E. coli N99, and OmpF-beta-endorphin fused peptide was synthesized and secreted into the culture medium through both the cytoplasmic and outer membranes. The OmpF signal peptide was cleaved correctly during the secretion, indicating that the export of the fused protein across the cytoplasmic membrane was dependent on the signal peptide. The secretion into the culture medium was apparently selective. Neither beta-lactamase nor alkaline phosphatase (both are periplasmic proteins) appeared in the culture medium in significant amounts. The mode of passage of the fused peptide across the outer membrane is discussed.     

Introduction of 6-formylcytidine into a Myb binding sequence

Single 6-formylcytidine was introduced into a oligonucleotide duplex (23 mers) as a substitute for thymidine in the Myb binding sequence of 3'-TTGAC-5'. The modified duplex showed Tm of 67 degrees C, which was six degrees lower than the Tm of the native duplex. Binding affinity of the 23-mers to the Myb protein was estimated by electrophoretic mobility shift assays, and the binding was almost completely abolished.