Chloroplast initiation factor 3 from Euglena gracilis. Identification and initial characterization.

A chloroplast ribosome dissociation factor (IF-3chl) has been identified in whole cell extracts of Euglena gracilis. This work represents the first report of an organellar ribosome dissociation factor. E. gracilis IF-3chl facilitates the dissociation of Escherichia coli ribosomes as demonstrated by sucrose density gradient analysis. Chloroplast IF-3 stimulates initiation complex formation on E. coli ribosomes with natural mRNA from the bacteriophage MS2. In addition, IF-3chl is effective in initiation complex formation with Euglena chloroplast or E. coli ribosomes in the presence of synthetic mRNA. IF-3chl is induced 12-fold by exposure of the cells to light. The chloroplast factor has been purified 30-fold by chromatography on DEAE-cellulose and phosphocellulose. The chromatographic properties of this factor differ considerably from those of prokaryotic ribosome dissociation factors.