Temperature-Sensitive Mutants of Escherichia coli K-12

Crosses were carried out at 34 C and 42 C between eight pairs of isogenic strains of Escherichia coli K-12. The donor and recipient of each pair carried the same mutation for temperature-sensitive deoxyribonucleic acid (DNA) synthesis; they differed only in the presence of F-lac in the donor and a spectinomycin-resistance marker in the recipient. A different temperature-sensitive mutation was present in each of the eight pairs, the eight temperature-sensitive mutations being located in at least two different genes. In all eight pairs, the transfer of F-lac occurred at high and equal rates at 34 C and 42 C, although vegetative DNA replication at 42 C was approximately 10-4 of that at 34 C. The transfer of F-lac at 42 C was accompanied in seven of the eight crosses by an equivalent amount ofDNA synthesis in excess of that observed in the unmated controls. The DNA synthesized during transfer at 42 C was characterized by equilibrium centrifugation in cesium chloride and by its sedimentation velocity in sucrose gradients. It was found to have a density and a molecular weight characteristic of F-lac DNA. A small proportion of the material labeled during transfer was recovered in the form of covalently closed DNA. It is concluded that vegetative replication of the chromosome and transfer replication of F are separate