Translation in Chloroplasts of Flowering Plants

Abstract

The chloroplast genome in flowering plants contains about 80 protein-coding genes. The chloroplast translational machinery, which is similar to that of Escherichia coli, reads the corresponding mRNAs. Translation initiation is critical to produce a correct protein. There are multiple possible initiation codons, either AUG or GUG, around an initiation region. Generally, cis-elements residing in a 5′-untranslated region and trans-acting factors are responsible for selection of genuine initiation codons. Unlike eubacterial mRNAs, a limited number of chloroplast mRNAs use Shine-Dalgarno-like sequences as their cis-elements, and many chloroplast mRNAs require specific trans-acting factors. As the chloroplast genome is compact, some genes (cistrons) partially overlap; namely, the start codon of a downstream cistron is located in front of the stop codon of its upstream cistron. In such a case, the downstream cistron is translated in a special manner called “translational coupling” and by an additional mechanism to produce the necessary amount of its product.