Bacillus subtilis Type I antitoxin SR6 Promotes Degradation of Toxin yonT mRNA and Is Required to Prevent Toxic yoyJ Overexpression.

/SR6 is the second type I toxin-antitoxin (TA) system encoded on prophage SPβ in the chromosome. The ORF specifying a 58 aa toxin is transcribed on a polycistronic mRNA under control of the promoter. The antitoxin SR6 is a 100 nt antisense RNA that overlaps at its 3' end and the downstream gene encoding a second, much weaker, toxin at its 5' end. SR6 displays a half-life of >60 min, whereas mRNA is less stable with a half-life of ≈8 min. SR6 is in significant excess over mRNA except in minimal medium with glucose. It interacts with the 3' UTR of mRNA, thereby promoting its degradation by RNase III. By contrast, SR6 does not affect the amount or half-life of mRNA. However, in its absence, a overexpression plasmid could not be established in suggesting that SR6 inhibits translation by directly binding to its ribosome-binding site. While the amounts of both RNA and SR6 were affected by vancomycin, manganese, heat-shock and ethanol stress as well as iron limitation, oxygen stress decreased only the amount of SR6.