way to maintain the stability of the plasmid. The F plasmid contains another homologous toxin-antitoxin system called srnB. The first type I toxin-antitoxin system to be found in gram-positive bacteria is the RNAI-RNAII system of the pAD1 plasmid in Enterococcus faecalis. Here, RNAI encodes a toxic protein Fst while RNAII is the regulatory sRNA. Chromosomal toxin-antitoxin systems In E. coli strain K-12 there are four long direct repeats (ldr) which encode short open reading frames of 35 codons organised in a homologous manner to the hok/sok system. One of the repeats encodes LdrD, a toxic protein which causes cell death. An unstable antisense RNA regulator (Rd1D) blocks the translation of the LdrD transcript. A mok homologue which overlaps each ldr loci has also been found. IstR RNA works in a similar system in conjunction with the toxic TisB protein. See also par stability determinant Addiction module Sib RNA SymR RNA PtaRNA1 RdlD RNA IstR RNA FlmB RNA RatA References Further reading Bacteriology Escherichia coli RNA antitoxins Cellular processes