Abstract
Chloramphenicol-sensitive, -resistant, and -temperature-sensitive R factors obtained from a chloramphenicol-resistant R factor (NR1) were characterized. The transfer frequency of the chloramphenicol-sensitive R factors was variable. The mechanisms underlying this variability are unknown. The chloramphenicol-sensitive-R-factor strains produced at a low frequency chloramphenicol-resistant revertants, some of which were temperature-sensitive to chloramphenicol. These R factors were not eliminated at 42 C indicating that the temperature-sensitive mutation is due to the mutation of the chloramphenicol resistance gene of the R factors. Survival of E. coli K12 carrying the chloramphenicol-temperature-sensitive R factor as a function of chloramphenicol concentration and of temperature was investigated. The growth of E. coli K12 carrying the R factors on chloramphenicol-containing agar plates was inhibited at 40 C or higher temperatures.
Two chloramphenicol-sensitive R factors were of high-transmissible mutant types, and transferred chromosomal genes at a higher frequency than the original R factor, NR1. The bacterial strains carrying the R factors were sensitive to phage f2. From one of them, we isolated episomes which had properties similar to the F factor and had lost all the drug-resistance genes. These episomes were designated as F-like factors. The F-like factors, probably derived from the R factor, were indistinguishable from the F factor except that the F-like factors transferred chromosomal genes by the mixed cultivation techniques at a slightly lower frequency than the F factor.
