Traditionally, programmed cell death (PCD) is associated with eukaryotic multicellular organisms. However, recently, PCD systems have also been observed in bacteria. Here we review recent research on two kinds of genetic programs that promote bacterial cell death. The first is mediated by mazEF, a toxin–antitoxin module found in the chromosomes of many kinds of bacteria, and mainly studied in Escherichia coli. The second program is found in Bacillus subtilis, in which the skf and sdp operons mediate the death of a subpopulation of sporulating bacterial cells. We relate these two bacterial PCD systems to the ways in which bacterial populations resemble multicellular organisms.

Summary Escherichia coli (E. coli) mazEF is a stress-induced toxin-antitoxin (TA) module. The toxin MazF is an endoribonuclease that cleaves single-stranded mRNAs at ACA sequences. Here, we show that MazF cleaves at ACA sites at or closely upstream of the AUG start codon of some specific mRNAs and thereby generates leaderless mRNAs. Moreover, we provide evidence that MazF also targets 16S rRNA within 30S ribosomal subunits at the decoding center, thereby removing 43 nucleotides from the 3′ terminus. As this region comprises the anti-Shine-Dalgarno (aSD) sequence that is required for translation initiation on canonical mRNAs, a subpopulation of ribosomes is formed that selectively translates the described leaderless mRNAs both in vivo and in vitro. Thus, we have discovered a modified translation machinery that is generated in response to MazF induction and that probably serves for stress adaptation in Escherichia coli.

The Escherichia coli mazEF module is one of the most thoroughly studied toxin–antitoxin systems. mazF encodes a stable toxin, MazF, and mazE encodes a labile antitoxin, MazE, which prevents the lethal effect of MazF. MazF is an endoribonuclease that leads to the inhibition of protein synthesis by cleaving mRNAs at ACA sequences. Here, using 2D-gels, we show that in E. coli, although MazF induction leads to the inhibition of the synthesis of most proteins, the synthesis of an exclusive group of proteins, mostly smaller than about 20 kDa, is still permitted. We identified some of those small proteins by mass spectrometry. By deleting the genes encoding those proteins from the E. coli chromosome, we showed that they were required for the death of most of the cellular population. Under the same experimental conditions, which induce mazEF-mediated cell death, other such proteins were found to be required for the survival of a small sub-population of cells. Thus, MazF appears to be a regulator that induces downstream pathways leading to death of most of the population and the continued survival of a small sub-population, which will likely become the nucleus of a new population when growth conditions become less stressful.

No abstract

mazEF is a stress-induced toxin-antitoxin module, located on the chromosome of Escherichia coli, that we have previously described to be responsible for programmed cell death in E. coli. mazF specifies a stable toxin, and mazE specifies a labile antitoxin. Recently, it was reported that inhibition of translation and cell growth by ectopic overexpression of the toxin MazF can be reversed by the action of the antitoxin MazE ectopically overexpressed at a later time. Based on these results, it was suggested that rather than inducing cell death, mazF induces a state of reversible bacteriostasis (K. Pederson, S. K. Christensen, and K. Gerdes, Mol. Microbiol. 45:501-510, 2002). Using a similar ectopic overexpression system, we show here that overexpression of MazE could reverse MazF lethality only over a short window of time. The size of that window depended on the nature of the medium in which MazF was overexpressed. Thus, we found "a point of no return," which occurred sooner in minimal M9 medium than it did in the rich Luria-Bertani medium. We also describe a state in which the effect of MazF on translation could be separated from its effect on cell death: MazE overproduction could completely reverse the inhibitory effect of MazF on translation, while not affecting the bacteriocidic effect of MazF at all. Our results reported here support our view that the mazEF module mediates cell death and is part of a programmed cell death network.Toxin-antitoxin systems have been found on the chromosomes of many bacteria (1,6,10,(17)(18)(19)21). In Escherichia coli such systems include mazEF (1,18,19), chpBIK (18), relBE (2, 7, 10), yefM-yoeB (4, 5, 11), and dinJ-yafQ (13). Each toxinantitoxin system consists of a pair of genes, of which the downstream gene encodes a stable toxin and the upstream gene encodes a labile antitoxin. The first toxin-antitoxin system carried on a bacterial chromosome that was described as regulatable and responsible for programmed cell death was the E. coli mazEF module (1), located in the relA operon (19). The product of mazF (MazF) is a stable toxin that inhibits translation by cleaving mRNA at a specific site(s) (6,26). In light of contradictory results from studies on this system, the mechanism of this cleavage is not yet well understood (6,22,26). The product of mazE (MazE) counteracts the action of MazF. Because MazE is a labile protein, degraded by the protease ClpAP (1), prevention of MazF-mediated death requires the continuous production of MazE. Thus, stressful conditions that prevent the expression of the chromosomally borne mazEF module should trigger cell death. Indeed, as predicted, we found several stressful conditions to cause mazEF-mediated cell death: (i) extreme amino acid starvation leading to the production of ppGpp (1, 9); (ii) inhibition of transcription and/or translation by antibiotics such as rifampin, chloramphenicol, and spectinomycin under specific growth conditions (25); (iii) inhibition of translation by the Doc protein of prophage P1 (15); (iv) DNA damage caused by th...

mazEF is a toxin-antitoxin module located on the Escherichia coli chromosome and that of some other bacteria, including pathogens. mazF specifies for a stable toxin, MazF, and mazE specifies for a labile antitoxin, MazE, that antagonizes MazF. MazF is a sequence-specific mRNA endoribonuclease that initiates a programmed cell death pathway in response to various stresses. The mazEFmediated death pathway can act as a defense mechanism that prevents the spread of bacterial phage infection, allowing bacterial populations to behave like multicellular organisms.