Toxic component of a type I toxin-antitoxin (TA) system. The sibB sRNA probably represses expression of ibsB mRNA, either by destabilizing the transcript and/or preventing its translation (Probable). Expression of the proteinaceous toxin is controlled by antisense sRNA SibB. None seen. An isbB overproducing strain cannot be made in the absence of the sibB gene. Part of the SIBb repeat region (formerly known as QUAD1b), encoded on the opposite strand from the sibB (formerly knowny as ryeD) RNA. Belongs to the Ibs toxic protein family.