Chloroplast and Cytoplasmic Ribosomes of Euglena : Selective Binding of Dihydrostreptomycin to Chloroplast Ribosomes

Abstract

Dihydrostreptomycin binds preferentially to chloroplast ribosomes of wild-type Euglena gracilis Klebs var. bacillaris Pringsheim. The K diss for the wild-type chloroplast ribosome-dihydrostreptomycin complex is 2 × 10 −7 M, a value comparable with that found for the Escherichia coli ribosome-dihydrostreptomycin complex. Chloroplast ribosomes isolated from the streptomycin-resistant mutant Sm 1 r BNgL and cytoplasmic ribosomes from wild-type have a much lower affinity for the antibiotic. The K diss for the chloroplast ribosome-dihydrostreptomycin complex of Sm 1 r is 387 × 10 −7 M, and the value for the cytoplasmic ribosome-dihydrostreptomycin complex of the wild type is 1,400 × 10 −7 M. Streptomycin competes with dihydrostreptomycin for the chloroplast ribosome binding site, and preincubation of streptomycin with hydroxylamine prevents the binding of streptomycin to the chloroplast ribosome. These results indicate that the inhibition of chloroplast development and replication in Euglena by streptomycin and dihydrostreptomycin is related to the specific inhibition of protein synthesis on the chloroplast ribosomes of Euglena .