TxpA-RatA toxin-antitoxin system
The TxpA/RatA toxin-antitoxin system was first identified in "Bacillus subtilis". It consists of a non-coding 222nt sRNA called RatA (RNA anti-toxin A) and a protein toxin named TxpA (Toxic protein A). RatA was discovered in intergenic regions of the "B. subtilis" genome, in a 728-nucleotide region between genes "yqdB" (later renamed "TxpA") and "yqbM". Initially, Affymetrix microarrays were used to detect transcription in this region, Northern blot experiments and mutation analysis were then employed to characterise the RNA transcript. "ratA" and "txpA" are transcribed convergently, and have an overlap of  75 base pairs at their 3' ends, providing an area of complementarity. The RatA transcript binds with the TxpA mRNA across the complementary region and the dsRNA is then degraded by an uncharacterised RNase E equivalent, preventing translation of the toxic TxpA protein. Genes "ratA" and "txpA" are found within a 48kb phage-like element called "skin". This element interrupts a gene for the sigma factor σ and is excised during sporulation. The toxin-antitoxin system contained within "skin" forces the inheritance of this element, which is acting as a selfish gene. The mechanism by which TxpA induces cell  lysis and death is unknown. TxpA is not similar enough to other proteins of known function to infer a related function, however it does have a suspected transmembrane region in its N-terminal, so it is possible that TxpA damages the integrity of the cell membrane, or blocks cell wall synthesis.