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

Abstract

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