The Eco29kI restriction-modification (R-M) system consists of two partially overlapping genes, eco29kIR, encoding a restriction endonuclease and eco29kIM, encoding methyltransferase. The two genes are thought to form an operon with the eco29kIR gene preceding the eco29kIM gene. Such an organization is expected to complicate establishment of plasmids containing this R-M system in naive hosts, since common logic dictates that methyltransferase should be synthesized first to protect the DNA from cleavage by the endonuclease. Here, we characterize the Eco29kI gene transcription. We show that a separate promoter located within the eco29kIR gene is sufficient to synthesize enough methyltransferase to completely modify host DNA. We further show that transcription from two intragenic antisense promoters strongly decreases the levels of eco29kIR gene transcripts. The antisense transcripts act by preventing translation initiation from the bicistronic eco29kIR-eco29kIM mRNA and causing its degradation. Both eco29kIM and antisense promoters are necessary for Eco29kI genes establishment and/or stable maintenance, indicating that they jointly contribute to coordinated expression of Eco29kI genes.
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