The Effect of Antibiotics on the Ultrastructure and Photochemical Activity of a Developing Chloroplast

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Abstract
The effects of the antibiotics chloramphenicol and cycloheximide on the ultrastructure of the chloroplast in greening cells of Euglena gracilis strain Z have been investigated. The rate of chloroplaat development in the presence of either antibiotic was closely related to that of chlorophyll production. Chloramphenicol, which at 10 mg/ml inhibits chlorophyll synthesis but not cell division, caused a marked inhibition of the development of chloroplast structure. The chloroplasts were smaller than those of untreated cells and contained a smaller number of internal lamellae. Most of these lamellae were not appressed and the results support the suggestion that chloramphenicol inhibits the synthesis of a protein responsible for the fusion of individual lamellae in the chloroplast. Measurement of the photochemical activity of chloroplasts isolated from chloramphenicol-treated cells showed that the photoreduction of NADP from water (photosystem I+II), photosystem II activity, and photosystem I activity were greatly inhibited when compared with chloroplasts from untreated cells. In contrast, cycloheximide at 2.5 to 5.0 μg/ml inhibited cell division but allowed the chloroplasts to develop after a lag phase, the length of which was related to the concentration of antibiotic employed. The number of lamellae per chloroplast and the degree of appression of the lamellae in chloroplasts of cycloheximide-treated cells and untreated cells were comparable. After 48-h illumination the photochemical activities of chloroplasts isolated from cycloheximide-treated cells were about 50 per cent of those of the untreated cells.

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