The Gene for Chloroplast Ribosomal Protein S16 is Located in Both Chloroplast and Nuclear Genomes: Expression of its Chloroplast Gene

The chloroplast was derived from a photosynthetic prokaryote similar to cyanobacteria. After endosymbiosis, a large number of genes in the ancestral chloroplast genome were transferred to the nuclear genome. Hence, present-day chloroplasts have only 100 to 150 genes. The gene for chloroplast 30S ribosomal protein S16 (rps16) is unique. This gene is found not only in the chloroplast genome but also in the nuclear genome in many flowering plants. Some of the nuclear-encoded S16 proteins were reported to be present in chloroplasts. Therefore, we analyzed the gene expression of chloroplast rps16 (cp-rps16) in tobacco plants. The RT-PCR analysis revealed that cp-rps16 is transcribed and spliced efficiently. The mature mRNA is accumulated in chloroplasts. Then, we analyzed translation of cp-rps16 mRNA using the chloroplast in vitro translation system that we have developed. We found that cp-rps16 mRNA was hardly translated, even though it has a typical Shine-Dalgarno sequence at the proper position. Further analysis using several chimeric mRNAs showed that the 5’ untranslated region of cp-rps16 mRNA caused such translation defect. However, cp-rps16 mRNA has translation activity in an E. coli in vitro translation system. This translation activity was inhibited by the addition of chloroplast S30 fraction. These results suggest that chloroplasts have some factors to repress or regulate the translation of cp-rps16 mRNA.