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]) has been detected in whole cell extracts and in isolated chloroplasts from Euglena gracilis. This factor can be detected by its ability to catalyze translocation on 70 S prokaryotic ribosomes such as those from E. 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. However, inhibitors of chloroplast protein synthesis such as streptomycin or spectinomycin have no effect on the induction of this factor by light. Furthermore, 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. These data strongly suggest that the genetic information for chloroplast EF-G resides in the nuclear genome, and that this protein is synthesized on cytoplasmic ribosomes prior to compartmentalization within the chloroplasts.