Nuclear-cytoplasmic interactions in the responses of Amoeba proteus and Amoeba discoides to streptomycin

Abstract

The growth rates of clones of various strains of Amoeba proteus, in growth limiting concentrations of streptomycin, have been measured. These experiments have shown the existence of naturally occurring inter-strain differences in resistance to this drug. The growth of sensitive strains is retarded in concentrations higher than 0.06 μg/ml. “Moderately resistant” and “resistant” strains can be differentiated, four times and six times, respectively, as resistant as the most sensitive strain. All strains of A. proteus investigated absorb the same amount of streptomycin per individual amoeba in standard conditions. The level of resistance of clones originating from heterotransfers between sensitive and resistant A. proteus strains, produced by the de Fonbrune technique of nuclear transplantation, is the same as the sensitive parent. Therefore the mechanisms conferring higher levels of resistance on some strains of A. proteus cannot be transmitted by either nuclear factors or cytoplasmic factors independently. A single strain of A. discoides was also investigated and found to be six times as resistant as the most sensitive A. proteus and to absorb only half as much streptomycin in similar conditions. Both heterotransfer strain types [resistant nucleus ( A. discoides) in sensitive cytoplasm ( A. proteus) and sensitive nucleus ( A. proteus) in resistant cytoplasm ( A. discoides)] are more resistant and absorb less streptomycin than the sensitive parent. No additive effects between factors from resistant A. proteus and A. discoides occurred in the relevant heterotransfer strains. It is concluded that independent nuclear and cytoplasmic factors determine the higher resistance of A. discoides, which is probably based on lower permeability to streptomycin, and that the cytoplasmic system is self-replicating.