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The mechanism of protein biosynthesis in chloroplasts is currently not well understood. However, in general, the process of translation in this organelle is thought to resemble that in prokaryotes more closely than that in the eukaryotic cell cytoplasm. For example, chloroplast ribosomes are 70 S in size and initiation occurs with formylmethionine (Kozak, 1983). In bacteria, the initiation of translation requires three auxiliary factors. IF-2 promotes the binding of fMet-tRNA to 30 S ribosomal subunits. IF-3 facilitates ribosome dissociation and is thought to proofread the initiation complex. IF-1 facilitates the action of both IF-2 and IF-3 (Gualerzi and Pon, 1990). In contrast, the eukaryotic cytoplasmic system requires about 10 factors for the formation of the completed initiation complex (Pain, 1986). Several of these are involved in the binding of Met-tRNA to the ribosome while others appear to be required for the interaction of the 40 S subunit with the 5’ cap structure on the message. |
