OPTIMIZATION OF PROTEIN-SYNTHESIS IN ISOLATED HIGHER-PLANT CHLOROPLASTS - IDENTIFICATION OF PAUSED TRANSLATION INTERMEDIATES

Protein synthesis in isolated, intact pea chloroplasts was optimized and compared to translation within chloroplasts in vivo. Many polypeptides labeled with [35S] methionine in isolated intact chloroplasts did not comigrate with polypeptides which were labeled within chloroplasts in vivo. Antibodies to the large subunit of ribulose-1,5-bisphosate carboxylase-oxygenase (EC 4.1.1.39) immunoprecipitated [35S]-labeled large subunit plus several lower-molecular-mass translation products of isolated chloroplasts. The lower-molecular-mass soluble translation products synthesized in pulse-labeled chloroplasts were converted into full-length large-subunit polypeptides during a subsequent chase period. This result suggests that many of the polypeptides observed in pulse-labeled chloroplasts are incomplete translation products which are the result of ribosome pausing at discrete points along chloroplast mRNAs. The pulse-chase technique was used to follow synthesis of the 34.5-kDa precursor of the psb A gene product and its processing to the mature 32-kDa polypeptide in isolated chloroplasts. Chloroplast translation profiles obtained using the pulse-chase assay were very similar to translation profiles obtained in vivo thus extending the utility of protein synthesis in isolated chloroplasts.