Function
Sib RNA regulates the expression of a toxic protein in a type I toxin-antitoxin system similar to that of hok/sok andldr-rdl genes. The constitutively expressed Sib transcript regulates the ibs (induction brings stasis) open reading frame which encodes a small 18-19 amino acid hydrophobic protein which slows growth at moderate levels of expression and is toxic when overexpressed. The ibs gene is on the opposite strand to sib and is completely complimentary, so the antisense-binding of Sib RNA with the ibs mRNA brings about dsRNA-mediated degradation.
When sib was deleted in multi-copy plasmids, the cells could not be maintained due to the toxicity of the unrepressed ibs protein. The toxicity mechanism of ibs protein is not fully understood, but a change in membrane potential upon over-expression of the protein suggests that interactions with membrane proteins or membrane insertion brings about cell death.
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