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SUMMARY The concentration of streptomycin (Sm) which selectively inhibits light-induced chloroplast development in non-dividing Euglena is the same as that which induces the loss of green-colony forming ability in dividing organisms. This concentration of Sm has no effect on division or viability. Chlorophyll synthesis is insensitive to streptomycin for the first 12 h of development but is strongly inhibited after this time. Between 72 and 96 h after the beginning of chloroplast development, Sm-treated organisms contain 10 yo of the chlorophyll and 24 of the carotenoids of algae developing in the absence of the antibiotic. The chlorophyll-to-carotenoid ratio in treated organisms at 72 to 96 h is 0.9, the same as is found at 12 h for organ- isms developing in the absence of Sm. In the presence of streptomycin, Euglena never develops the ability to fix CO, photosynthetically, although CO, fixation after I 2 h of development in the absence of the antibiotic can be readily detected. At I 2 h of chloroplast development the following parameters are at comparable levels in Sm-treated and untreated organisms: the bound forms of chlorophyll, con- centration of cytochrome 552, the activities of ribulose diphosphate carboxylase, NADP-triose phosphate dehydrogenase, the enzymes converting ribose-yphos- phate to ribulose diphosphate, and photosystem I1 activity measured as dye reduction.

Events Surrounding the Early Development of Euglena Chloroplasts: Experiments with Streptomycin in Non-dividing Cells