A novel immune system against bacteriophage infection using complementary RNA (micRNA).

The "operon' theory of gene regulation, in which protein repressor molecules bind to the operator site of a gene to prevent its transcription, is now well established. Recently, however, cases have been discovered in which gene expression is regulated by complementary RNA molecules that are able to bind to the transcripts of particular genes and consequently prevent their translation. For example, the synthesis of OmpF protein (a major outer membrane protein) in Escherichia coli is regulated by a short RNA complementary to a region of ompF RNA encompassing the "Shine-Dalgarno' sequence and the translation initiation codon. This RNA has been termed micRNA (messenger-RNA-interfering complementary RNA), and its discovery has prompted us to construct an artificial micRNA system designed to regulate gene expression in E. coli. A given target gene can be repressed by artificially producing an RNA (micRNA) complementary to the mRNA encoded by that gene. A micRNA system has also been used successfully in tissue-cultured mammalian cells. The use of artificial micRNAs to specifically regulate individual genes has great potential as a novel cellular immune system for blocking bacteriophage or virus infection. Here, we report that on induction of micRNAs directed against the coat protein and/or the replicase of the E. coli bacteriophage SP, phage proliferation was effectively prevented. We propose that the micRNA immune system provides an effective means of preventing viral infection as well as the expression of harmful genes in both prokaryotes and eukaryotes.