Effect of Streptomycin on Chlorophyll Accumulation in Euglena gracilis*

Bleaching (interference with chlorophyll accumulation) by streptomycin (SM) is more effective when dark‐grown Euglena gracilis cells are exposed to SM in the dark for several days than when exposure of dark‐ or light‐grown cells is exclusively in light. Cells are also bleached when exposed to SM while lacking both chlorophyll and chloroplasts (i.e., exposed to SM in the dark and subcultured to SM‐free medium before transfer to light). Bleaching therefore involves inhibition of the formation of a plastid or pigment precursor, rather than breakdown of chlorophyll or interference with replication of mature chloroplasts. Bleaching by SM B prevented by addition to the medium of any of several fin cations, including Mg. Therefore it is proposed that bleaching results from chelation of Mg by SM. thus blocking a Mg‐requiring step in chlorophyll synthesis. Disruption by SM of chloroplast structure and/or development is considered a secondary consequence of the bleaching action, resulting from the essentiality of intact chlorophyll molecules for normal chloroplast structure.