Figure 1. MazF RNase toxin promotes loss of most proteins but selective increase of some small proteins with functions in the MazEF death-and-survival program. When MazF RNase is unmasked by the loss of MazE antitoxin, which binds and inactivates MazF, most cellular mRNAs are degraded, and rapid loss of most proteins occurs. Some mRNAs are protected, and Amitai et al. show that some small (less than 20 kDa) proteins are increased in abundance. Some of these proteins promote the death of most of the cells in the population, whereas others promote the survival of a small cell subpopulation in the MazEF death-and-survival program. doi:10.1371/journal.pgen.1000418.g001 in their mRNAs, implying that some mechanism, which is not yet understood, protects these mRNAs specifically. Six of the up-regulated proteins were required for MazF-dependent death, sug- gesting an active death mechanism. One of the ‘‘death proteins,’’ the ClpP protease, was already known to degrade the MazE antitoxin, acting upstream in the pathway that ultimately unleashes the MazF RN- ase. It will be interesting to examine whether the rest of the death proteins also allow MazF action, or whether death requires something more than destruction of most of the cell’s mRNAs. Perhaps even more surprisingly, three of the 13 up-regulated proteins, plus another two candidate proteins they tested, are required for survival of a small subpopula- tion of the cells when most of the cells are killed. This is the first indication that there are ‘‘survival proteins’’ that actively protect References 1. Shapiro JA (1998) Thinking about bacterial populations as multicellular organisms. Annu Rev Microbiol 52: 81–104. 2. Bassler BL, Losick R (2006) Bacterially speaking. Cell 125: 237–46. 3. Dubnau D, Losick R (2006) Bistability in bacteria. Mol Microbiol 61: 564–72. 4. Veening JW, Smits WK, Kuipers OP (2008) Bistability, epigenetics, and bet-hedging in bacte- ria. Annu Rev Microbiol 62: 193–210. 5. Gonzalez C, Hadany L, Ponder RG, Price M, Hastings PJ, et al. (2008) Mutability and impor- a subpopulation when the main population dies. This is reminiscent of bistable popu- lations. In this case one (large) subpopula- tion is slated for death while a second smaller subpopulation survives, as if there are both death and survival programs activated (in different cells) by MazF. The implication is that the main population is killed so that the subpopulation may survive, supporting the view that MazEF- mediated death serves a multicellular or at least social purpose. The present study does not distinguish which cells, surviving or dying, make which proteins. Previous work showed that the death program requires cell–cell communication. A secreted pentapeptide, which signals high cell numbers and cellular stress, must be sensed for the program to run [14]. An intriguing question raised here is whether the death proteins are suicide proteins made by the tance of a hypermutable cell subpopulation that produces stress-induced mutants in Escherichia coli. PLoS Genet 4: e1000208. doi:10.1371/journal. pgen.1000208. 6. Sung HM, Yasbin RE (2002) Adaptive, or stationary-phase, mutagenesis, a component of bacterial differentiation in Bacillus subtilis. J Bacteriol 184: 5641–5653. 7. Sharov AA, Weiner L, Sharova TY, Siebenhaar F, Atoyan R, et al. (2003) Noggin overexpression inhi- bits eyelid opening by altering epidermal apoptosis and differentiation. Embo J 22: 2992–3003. dying cells or assassins sent from those surviving? Two of the survival proteins protect cells against oxidative damage and can be understood in the context of this group’s previous finding that one of the ways that MazF promotes cell death requires oxida- tive stress—i.e., it can be quenched by any of several means of scavenging reactive oxygen species [15]. These survival pro- teins are presumably made and used in the surviving cells. How and where the remaining survival proteins work remains to be revealed. Bacteria lead more coordinated lives than bacterial geneticists initially appreci- ated. Viewed as groups of individuals, bacteria would seem to be enacting Hamlet- or Macbeth-like tragedies with systems like MazEF. But they may be viewed more usefully, though no less dramatically, as ‘‘simply’’ multicellular. 8. Ellermeier CD, Hobbs EC, Gonzalez-Pastor JE, Losick R (2006) A three-protein signaling path- way governing immunity to a bacterial cannibal- ism toxin. Cell 124: 549–559. 9 . A m i t a i S , K o l o d k i n - G a l I , H a n a n y a - Meltabashi M, Sacher A, Engelberg-Kulka H (2009) Escherichia coli MazF leads to the simulta- neous selective synthesis of both ‘‘death proteins’’ and ‘‘survival proteins.’’ PLoS Genet 5: e1000390. doi:10.1371/journal.pgen.1000390. 10. Jorgensen MG, Pandey DP, Jaskolska M, Gerdes K (2009) HicA of Escherichia coli defines PLoS Genetics | www.plosgenetics.org 2 March 2009 | Volume 5 | Issue 3 | e1000418