Although toxin-antitoxin gene cassettes were first found in plasmids, recent database mining has shown that these loci are abundant in free-living prokaryotes, including many pathogenic bacteria. For example, Mycobacterium tuberculosis has 38 chromosomal toxin-antitoxin loci, including 3 relBE and 9 mazEF loci. RelE and MazF are toxins that cleave mRNA in response to nutritional stress. RelE cleaves mRNAs that are positioned at the ribosomal A-site, between the second and third nucleotides of the A-site codon. It has been proposed that toxin-antitoxin loci function in bacterial programmed cell death, but evidence now indicates that these loci provide a control mechanism that helps free-living prokaryotes cope with nutritional stress.

The mechanism of prokaryotic chromosome segregation is not known. MreB, an actin homolog, is a shapedetermining factor in rod-shaped prokaryotic cells. Using immuno¯uorescence microscopy we found that MreB of Escherichia coli formed helical ®laments located beneath the cell surface. Flow cytometric and cytological analyses indicated that MreB-depleted cells segregated their chromosomes in pairs, consistent with chromosome cohesion. Overexpression of wildtype MreB inhibited cell division but did not perturb chromosome segregation. Overexpression of mutant forms of MreB inhibited cell division, caused abnormal MreB ®lament morphology and induced severe localization defects of the nucleoid and of the oriC and terC chromosomal regions. The chromosomal terminus regions appeared cohered in both MreBdepleted cells and in cells overexpressing mutant forms of MreB. Our observations indicate that MreB ®laments participate in directional chromosome movement and segregation.

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The parB region of plasmid R1 encodes two genes, hok and sok, which are required for the plasmid‐stabilizing activity exerted by parB. The hok gene encodes a potent cell‐killing factor, and it is regulated by the sok gene product such that cells losing a parB‐carrying plasmid during cell division are rapidly killed. Coinciding with death of the host cell, a characteristic change in morphology is observed. Here we show that the killing factor encoded by the hok gene is a membrane‐associated polypeptide of 52 amino acids. A gene located in the Escherichia coli relB operon, designated relF, is shown to be homologous to the hok gene. The relF gene codes for a polypeptide of 51 amino acids, which is 40% homologous to the hok gene product. Induced overexpression of the hok and relF gene products results in the same phenomena: loss of cell membrane potential, arrest of respiration, death of the host cell and change in cell morphology. The parB region and the relB genes were cloned into unstably inherited oriC minichromosomes. Whereas the parB region also conferred a high degree of genetic stability to an oriC minichromosome, the relB operon (with relF) did not; therefore the latter does not appear to ‘stabilize’ its replicon (the chromosome). The function of the relF gene is not known.