The nucleotide sequence was determined of the region upstream of the
mukB gene of
Escherichia coli. Two new genes were found, designated
kicA and
kicB (killing of cell); the gene order is
kicB-kicA-mukB. Promoter activities were detected in the regions immediately upstream of
kicB and
kicA, but not in front of
mukB. Gene disruption experiments revealed that the
kicA disruptant was nonviable, but the
kicB-disrupted mutant and the mutant lacking both the
kicB and
kicA genes were able to grow. When
kicA disruptant cells bearing a temperature-sensitive replication plasmid carrying the
kicA
+ gene were grown at 30° C and then transferred to 42° C, the mutant cells gradually lost colony-forming ability, even in the presence of a
mukB
+ plasmid. Rates of protein synthesis, but not of RNA or DNA synthesis, fell dramatically during incubation at 42° C. These results suggested that the
kicB gene encodes a killing factor and the
kicA gene codes for a protein that suppresses the killing function of the
kicB gene product. It was also demonstrated that
KicA and KicB can function as a post-segregational killing system, when the genes are transferred from the
E. coli chromosome onto a plasmid.
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