Abstract
Tobacco chloroplast genome contains about 80 protein genes that are expressed by chloroplast gene expression system. Many chloroplast mRNAs lack SD-like sequences, which are found in the 5'-untranslated region of most of bacterial mRNAs. A common or specific trans-acting factor is thought to be involved in translational regulation of such chloroplast mRNAs but translational regulation of chloroplast mRNAs is still unclear. In the present study, in vitro translation analysis of mutagenized psaA and psbD mRNAs, which carry an SD-like sequence at bacterial consensus position, revealed that translation initiation of these mRNAs is highly dependent on their SD-like sequences and that relatively short region of the 5'-UTR was sufficient for efficient translation initiation. In the case of mRNAs such as ycf4 and psbA that lack SD-like sequences, relatively long region of the 5'-UTR was required for efficient translation initiation of these mRNAs. Mechanism of translation initiation in chloroplasts will be discussed.
