Isolation and characterization of a copy mutant of the bacteriocinogenic plasmid Clo DF13

After nitrosoguanidine mutagenesis, strain Escherichia coli P678-54, bacteriocinogenic for Clo DF13, yielded a mutant strain that showed an enhanced bacteriocin production. The results from conjugation experiments indicated that the mutation, responsible for the enhanced bacteriocin production, is located on the Clo DF13 plasmid. The following properties of strains harboring the mutant Clo DF13 plasmid could be observed. (i) The bacteriocin production in these strains can be further enhanced at least fourfold by mitomycin C. (ii) The fraction of spontaneously induced cells, as revealed by lacunae experiments, in cultures of these strains is about nine times higher than in cultures of wild-type Clo DF13-harboring strains. (iii) Chromosomeless minicells from strain P678-54 harboring the mutant Clo DF13 plasmid synthesize about six times more deoxyribonucleic acid, ribonucleic acid, and protein as compared to wild-type Clo DF13-harboring minicells. (iv) Analysis of this mutant Clo DF13-specific ribonucleic acid and protein on polyacrylamide gels revealed mainly the same ribonucleic acid and polypeptide species as synthesized by the wild-type Clo DF13 minicells, but in larger amounts (Kool et al., 1974). (v) Segregation experiments, using a strain with temperature-sensitive polymerase I, show that mutant Clo DF13-harboring cells contain an average of 70 Clo DF13 copies per cell, whereas wild-type Clo DF13-harboring cells contain only about 10 Clo DF13 copies per cell. The data presented in this paper indicate that the mutation on the Clo DF13 plasmid leads to an altered control of Clo DF13 replication and results in an enhanced number of Clo DF13 copies per cell. As a secondary effect, this enhanced number of Clo DF13 copies enhances the probability of "spontaneous" induction per cell. Since the mutation is plasmid specific and affects the number of plasmid copies produced, one can conclude that the Clo DF13 plasmid is not dependent solely on chromosomal information, but that at least plasmid base sequences are involved in Clo DF13 plasmid replication.