Nuclear-Transfer Studies on Induced Increases in Streptomycin Resistance of Amoeba proteus

SYNOPSIS. The responses of a strain of Amoeba proteus to conditions which could give rise to induced increases in resistance to streptomycin have been investigated. An indirect selection method, based on survival time in high concentrations of streptomycin, was devised. All attempts to obtain substrains with enhanced sensitivity or enhanced resistance by indirect methods were unsuccessful. It was therefore concluded that increases in resistance observed as a result of other treatments were likely to be the result of adaptation of a majority of individuals, rather than selection of pre-existing resistant variants. Direct treatment with regularly increased concentrations of streptomycin led to a 10-fold increase in resistance of large cultures. Nuclear transfers were made by the de Fonbrune technique to produce resistant nucleus/normal cytoplasm, and normal cytoplasm/resistant nucleus combinations. Examination of the descendants of these 'crosses' showed that the resistance was nuclear dependent and not influenced by the cytoplasm. The resistance proved transitory, disappearing slowly during continued culture without further contact with streptomycin.
These results are discussed with reference to possible mechanisms responsible for the increase in streptomycin resistance and its mode of origin.