a novel family of translation-independent mRNA interferases in bacteria and archaea. J Bacteriol 191: 1191–1199. 11. Magnuson RD (2007) Hypothetical functions of toxin-antitoxin systems. J Bacteriol 189: 6089– 6092. 12. Van Melderen L, Saavedra de Bast M (2009) Bacterial toxin-antitoxin systems: more than selfish entities? PLoS Genet, In press. 13. Zhang Y, Zhang J, Hoeflich KP, Ikura M, Qing G, et al. (2003) MazF cleaves cellular mRNAs specifically at ACA to block protein synthesis in Escherichia coli. Mol Cell 12: 913–923. 14. Kolodkin-Gal I, Hazan R, Gaathon A, Carmeli S, Engelberg-Kulka H (2007) A linear pentapeptide is a quorum-sensing factor required for mazEF- mediated cell death in Escherichia coli. Science 318: 652–655. 15. Kolodkin-Gal I, Sat B, Keshet A, Engelberg- Kulka H (2008) The communication factor EDF and the toxin-antitoxin module mazEF determine the mode of action of antibiotics. PLoS Biol 6: e319. doi:10.1371/journal.pbio.0060319. PLoS Genetics | www.plosgenetics.org 3 March 2009 | Volume 5 | Issue 3 | e1000418