クックトミート培地に保存中に見られたR12(CM・SM・SA)因子の自然消失と非伝達性変異

Escherichia coli K-12 F- harboring R12 (CM. SM. SA) factor was stored in cooked meat medium at room temperature. After 3 years, it was examined for loss of R factor, as well as the genetic properties of this factor. It was found that 90.5% of surviving cells had lost their R factor to be sensitive to chloramphenicol (CM), streptomycin (SM), and sulfanilamide (SA). In the other surviving cells retaining their R factor, no segregation was noticed among resistance markers, So long as examined. Two derivative R factors, R12-5 and R12-22, were proveed to be nontransferable by conjugation, indicating that transferable (tra+) R12 factor had mutated to tra- R factor by storage in cooked meat medium for along time. From the results of a curing experiment and transduction to the rec- strain, it was clarified that the tra- R factors, R12-5 and R12-22, existed extrachromosomally and were replicated (rep+) autonomously in their host. Genetic studies revealed that R12-5 and R12-22 factors were ifm+ (inhibition of F-mating) and irs+ (interference of R factor superinfection), as well as the original R12 factor was. Reversion took place from tra- to tra+ at a frequency of 8.0 to 8.8×10-10, indicating that these were point mutants of the tra loci. The E. coli K-12 strain carrying either tra- R12-5 or tra- R12-22 acquired an ability to transfer its nontransferable R factor by complementation in cooperation with F factor, chromosomal F of the Hfr strain, or with transferable R factor.