Effects of cycloheximide and of chloramphenicol on DNA synthesis in Euglena gracilis

Abstract

1. The independence of chloroplasts in synthesis of their DNA has been examined in Euglena gracilis by 32Plabeling of the DNA and with the aid of the inhibitors, chloramphenicol and cycloheximide. Strain Z, containing chloroplast and mitochondrial DNA, and strain W3 BUL, containing mitochondrial DNA, were used. 2. Chloramphenicol has no effect on the labeling or on the distribution of chloroplast and mitochondrial DNA relative to nuclear DNA for at least 5.5 generations. Cell division is slowed in strain Z but is unaffected in strain W3 BUL. Since chloramphenicol inhibits protein synthesis on Euglena chloroplast ribosomes, continued synthesis of chloroplast DNA and continued cell division suggest that all unstable proteins essential for synthesis of chloroplast DNA are made on cytoplasmic ribosomes, or all proteins required for this synthesis turn over slowly. 3. After growth in the presence of cycloheximide, the specific activity of the nuclear DNA is 35 to 40 % of that of the mitochondrial DNA and 50 % of that of the chloroplast DNA. Under these conditions cell division is halted. Since control cultures did not show increased turnover of extranuclear DNA, synthesis of chloroplast and mitochondrial DNA must continue after synthesis of nuclear DNA has been virtually stopped by cycloheximide.