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.

Escherichia coli relB mutants react to amino acid starvation by several abnormal responses, including accumulation of a translational inhibitor. We have isolated a relB‐complementing plasmid from the Clarke and Carbon E. coli DNA library. From this plasmid we sequenced a 2140‐bp segment which included the relB gene by the following two criteria: (i) it complements chromosomal relB mutations, (ii) the corresponding DNA segment cloned from chromosomal DNA of three relB mutants was defective in relB complementation. All three mutations fell within an open reading frame of 79 amino acids. A polypeptide of 9 kd compatible with this open reading frame was synthesized in maxicells and is in all probability the product of the relB gene. By nuclease S1 mapping we have determined the transcription start and stop of an 870 base transcript of the relB gene.