Function of micF as an antisense RNA in osmoregulatory expression of the ompF gene in Escherichia coli

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Abstract
To analyze the function of micF as an antisera RNA in the osmoregulatory expression of the ompF gene in Escherichia coli, we performed two experiments. In the first experiment, two strains were constructed in which the transcription initiation site of the ompF gene and the transcription termination site of the micF gene were separated by 186 and 4,100 base pairs, respectively, on the chromosome. These two strains showed almost the same profile of ompF expression as the wild-type strain in which the two genes are separated by 10(6) base pairs. When a high-copy-number plasmid carrying the micF gene was introduced into these strains, ompF expression was completely repressed, whereas no repression was observed with a low-copy-number plasmid carrying the micF gene. These results indicate that the distance between the two genes on the chromosome is not critical for the function of micF. In the second experiment, expression of the ompF gene was examined by pulse-labeling in both the micF+ and the micF deletion strains. Upon a shift from a low- to a high-osmolarity medium, suppression of OmpF protein synthesis occurred more quickly and more extensively in the micF+ strain than in the micF deletion strain. The steady-state synthesis of the OmpF protein was also completely suppressed in the micF+ strain in the high-osmolarity medium, whereas the suppression was incomplete in the micF deletion strain. From these results we conclude that (i) the micF gene contributes to the fast and complete response of the OmpF synthesis to the medium osmolarity, and that (ii) the distance between the micF and ompF genes on the chromosomes is not critical for the function of the micF gene. The results suggest, rather, that the ratio of the copy numbers of the two genes is critical for the function of the micF gene.