Abstract
Temperature-sensitive (Ts) mutant plasmids isolated from tetracycline resistance R plasmid pSC101 were investigated for their segregation kinetics and deoxyribonucleic acid (DNA) replication. The results fit well with the hypothesis that multiple copies of a plasmid are distributed to daughter cells in a random fashion and are thus diluted out when a new round of plasmid DNA replication is blocked. When cells harboring type I mutant plasmids were grown at 43 degrees C in the absence of tetracycline, antibiotic-sensitive cells were segregated after a certain lag time. This lag most likely corresponds to a dilution of plasmids existing prior to the temperature shift. The synthesis of plasmid DNA in cells harboring type I mutant plasmids was almost completely blocked at 43 degrees C. It seems that these plasmids have mutations in the gene(s) necessary for plasmid DNA replication. Cells haboring a type II mutant plasmid exhibited neither segregation due to antibiotic sensitivity nor inhibition of plasmid DNA replication throughout cultivation at high temperature. It is likely that the type II mutant plasmid has a temperature-sensitive mutation in the tetracycline resistance gene. Antibiotic-sensitive cells haboring type III mutant plasmids appeared at high frequency after a certain lag time, and the plasmid DNA synthesis was partially suppressed at the nonpermissive temperature. They exhibited also a pleiotrophic phenotype, such as an increase of drug resistance level at 30 degrees C and a decrease in the number of plasmid genomes in a cell.