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We have confirmed and extended the observation of Terawaki et al. that the R factor, Rts1, alters the growth of its host at 42 C. In all media tested there was a period during which total cell numbers increased linearly, while viable counts remained constant. During this period the rate of precursor incorporation per cell particle into deoxyribonucleic acid, ribonucleic acid, and protein declined steadily. These patterns were a consequence of the accumulation of increasing numbers of cells which had lost colony-forming ability. A temperature shiftdown experiment showed that the colony formers could, after a lag, go on to divide normally, whereas most of the noncolony formers could not undergo even a limited number of divisions after shiftdown. The number of normal divisions which occurred after shiftup of Rts1 cells to 42 C was medium dependent. In rich medium there were, on the average, two or three doublings; in glucose medium, one; and in glycerol medium, only a fraction of a doubling. Even in glucose medium, however, no increase in viable counts was observed during growth at 42 C if the cells were first starved for glucose for 1 h at 42 C. A temperature shiftdown from 42 C to 27 C during glucose starvation reversed the effect of starvation at 42 C alone. These results are consistent with the hypothesis that the thermosensitive Rts1 component(s) responsible for the host effects is present at permissive temperature, but can undergo a reversible temperature-induced alteration which then interferes with some essential host function. The detrimental effects of this R factor on its host were also reflected in a heightened sensitivity to kanamycin and actinomycin D at 42 C. Electron microscope observations revealed changes in the appearance of the cell membrane. Membranous invaginations were noted at discrete sites in the cell.

Host Cell Growth in the Presence of the Thermosensitive Drug Resistance Factor, Rts1