THE PLEIOTROPIC ts15 MUTATION OF E. COLI IS AN IS1 INSERTION IN THE rho STRUCTURAL GENE

Rho protein regulates transcription termination in E. coli. Some of the temperature-sensitive mutants defective in Rho protein, e.g., ts15, show remarkable pleiotropic phenotypes. The ts mutations map between the ilv and cya loci on the E. coli chromosome. We have cloned the gene that restores the wild-type phenotypes of these mutants. Genetic and biochemical characterizations have shown that the cloned DNA segment carries the structural gene for the Rho polypeptide. Analysis of the rhots15 mutation has revealed the presence of an IS1 insertion in the carboxy terminal segment of the rho cistron, thereby truncating the 52-kilodalton (kd) Rho polypeptide to a 50-kd size and also making it thermolabile. This provides an example of how an IS1 insertion mutation can cause a TS phenotype. We have also shown that the multiple phenotypes of the mutant cell, including the temperature sensitivity, are caused by a single mutation (rhots15::IS1) in the rho structural gene. How a rho structural gene mutation may cause such pleiotropy is discussed.