EVIDENCE FOR THE NUCLEAR LOCATION OF THE GENE FOR CHLOROPLAST ELONGATION FACTOR-G

The chloroplast protein synthesis factor responsible for the translocation step of polypeptide synthesis on chloroplast ribosomes (chloroplast elongation factor G [EF-G]) was detected in whole cell extracts and in isolated chloroplasts from Euglena gracilis. This factor was detected by its ability to catalyze translocation on 70 S prokaryotic ribosomes such as those from Escherichia coli. Chloroplast EF-G is present in low levels when Euglena is grown in the dark and can be induced more than 20-fold when the organism is grown in the light. The induction of this factor by light is inhibited by cycloheximide, a specific inhibitor of protein synthesis on cytoplasmic ribosomes. Inhibitors of chloroplast protein synthesis such as streptomycin or spectinomycin have no effect on the induction of this factor by light. Chloroplast EF-G can be partially induced by light in an aplastidic mutant (strain W3BUL) which has neither significant plastid structure nor detectable chloroplast DNA. The genetic information for chloroplast EF-G probably resides in the nuclear genome and this protein is synthesized on cytoplasmic ribosomes prior to compartmentalization within the chloroplasts.