A locus affecting nucleoid segregation in Salmonella typhimurium.

Thirteen temperature-sensitive lethal mutations of Salmonella typhimurium map near metC at 65 min and form the clmF (conditional lethal mutation) locus. The mutations in this region were ordered by three-point transduction crosses. After a shift to the nonpermissive temperature, many of these clmF mutants failed to complete the segregation of nucleoids into daughter cells; daughter nucleoids appeared incompletely separated and asymmetrically positioned within cells. Some clmF mutants showed instability of F' episomes at permissive growth temperatures yet showed no detectable defect with smaller multicopy plasmids such as pSC101 or pBR322. In addition, many of the clmF mutants rapidly lost viability yet continued DNA replication at the nonpermissive temperature. These results suggest that the clmF locus encodes at least one indispensable gene product that is required for faithful partitioning of the bacterial nucleoid and F-plasmid replicons.