Toxic component of a type I toxin-antitoxin (TA) system. Overexpression causes cessation of growth. The sibA sRNA probably represses expression of ibsA mRNA, either by destabilizing the transcript and/or preventing its translation. Expression of the proteinaceous toxin is controlled by antisense sRNA SibA. None seen. Part of the SIBa repeat region (formerly known as QUAD1a), encoded on the opposite strand from the sibA (formerly known as ryeC) RNA. Belongs to the Ibs toxic protein family.