Studies on translation mechanisms of plastid mRNAs

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We found following molecular mechanisms of translation in chloroplasts using in vitro systems from isolated tobacco chloroplasts.(1) Determination of initiation codons.The tobacco chloroplast ndhD mRNA has three possible intiation codons. We determined the AUG triplet created from ACG by RNA editing the initiation codon. Then, GUG was found to be the initiation codon of psbC mRNA which has AUG and GUG triplets in-frame.(2) Efficiency of initiation codonsThe efficiency of translation was determined to be in the order of AUG, GUG and UUG.(3) Translation of dicistronic mRNAsThe psaC-ndhD mRNA was found not to be translated but did after intercistronic cleavage. On the other hand, the psbD-psbC mRNA was translated as it is.(4) Effect of the Shine-Dalgarno (SD) sequenceThe SD sequence was found to be functional only when this is located 5-12 nucleotide upstream from AUG.(5) Relationship between RNA editing and translationWe have succeeded in developing an in vitro RNA editing system from isolated tobacco chloroplasts, and proved that the edited AUG functions as an initiation codon.