Abstract
yonT/SR6 is the second type I toxin-antitoxin (TA) system encoded on prophage SPβ in the B. subtilis chromosome. The yonT ORF specifying a 58 aa toxin is transcribed on a polycistronic mRNA under control of the yonT promoter. The antitoxin SR6 is a 100 nt antisense RNA that overlaps yonT at its 3′ end and the downstream gene yoyJ encoding a second, much weaker, toxin at its 5′ end. SR6 displays a half-life of >60 min, whereas yonT mRNA is less stable with a half-life of ≈8 min. SR6 is in significant excess over yonT mRNA except in minimal medium with glucose. It interacts with the 3′ UTR of yonT mRNA, thereby promoting its degradation by RNase III. By contrast, SR6 does not affect the amount or half-life of yoyJ mRNA. However, in its absence, a yoyJ overexpression plasmid could not be established in Bacillus subtilis suggesting that SR6 inhibits yoyJ translation by directly binding to its ribosome-binding site. While the amounts of both yonT 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.
Highlights
To date, six types of toxin–antitoxin systems are known
We report on the second type I TA system from the B. subtilis chromosome that
We report on the second type I TA system from the B. subtilis chromosome that is is encoded on the SPβ prophage
Summary
Six types of toxin–antitoxin systems are known (reviewed in [1,2]). All these systems comprise two genes, one encoding a stable toxin, the other an unstable antitoxin that neutralizes the toxin action. Type I antitoxins interact with their toxin mRNAs that mostly code for small hydrophobic peptides recruited to the cell membrane, and for RNA or DNA cleaving enzymes The antitoxin is a cis-encoded antisense RNA It was speculated that toxin expression might be advantageous upon stress, as shown for some type II TA systems The secondary structures of bsrE RNA and bsrG RNA are very similar, and binding between toxin mRNA and the cognate antitoxin occurs in both systems via three subsequent steps [22]. SR5 is—in contrast to SR4—a monofunctional antitoxin that only promotes degradation of its bsrE toxin mRNA by recruiting. A BLAST search for homologues of yonT, yoyJ and sr revealed that yonT and sr are encoded in five Bacillus species, whereas the yoyJ gene or its promoter was mutated, so that no functional protein can be expressed
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