Specific Inhibition of Chloroplast Replication in Euglena Gracilis by Nalidixic Acid

In Euglena gracilis the synthesis of the chloroplast from its precursor, the proplastid, is separated in time from the division or replication of the organelle which occurs just prior to cell division (10, 12). The proplastids are only found in darkgrown cells and rapidly form chloroplasts when such cells are exposed to light. The proplastids divide during dark growth. Ultraviolet irradiation inhibits proplastid division or replication but does not interfere with the proplastid's ability to synthesize chloroplasts (12). Chloroplast replication is also inhibited by ultraviolet irradiation (8). Euglena, then, is an excellent organism for the study of chloroplast synthesis and replication. Nalidixic acid (-ethyl 1-4 dihydro-7-methyl- 4 -oxo- 1,8 naphthyridine-3-carboxylic acid) has been shown to be a specific inhibitor of DNA replication in certain bacteria, but to be bactericidal only if RNA or protein synthesis is allowed to proceed (4-6). We have found that this compound is a specific inhibitor of chloroplast replication in Euglena gracilis and that it has no effect on chloroplast synthesis or cell division. Proplastid replication is also inhibited but to a lesser degree. Inhibition of chloroplast synthesis markedly reduces the effect of nalidixic acid on chloroplast replication.