Transgenic study of chloroplast translocon gene regulation in Arabidopsis thaliana

The majority of chloroplast proteins is encoded by the nuclear genome and must be accurately imported to plastids. The translocons on the outer envelope and inner envelope membrane of chloroplasts (the Toe and Tic proteins) play key roles in the machinery of protein import into the chloroplast. Among the Toe and Tic components identified in Arabidopsis thaliana, atToc33 and atToc34 are paralogous to pea psToc34 and play important roles in pre-protein recognition. The expression of atToc33 and atToc34 genes must be properly regulated, or their gene products will not be correctly integrated to their destination to allow their function. To reveal the regulatory mechanism of atToc33 and atToc34 gene expression, transgenes containing various lengths of the upstream regulatory sequences of atToc33 and atToc34 genes and GUS coding sequence were transferred into wild-type Arabidopsis. We found that the atToc33 and atToc34 genes are preferentially expressed in leaves and roots, respectively. Furthermore, atToc34 is also regulated in an age-dependent manner. Finally, the leader intron in the 5' UTR of both genes up-regulates the gene expression in mature plants. These data suggest that atToc33 and atToc34 play distinct roles in chloroplast function and development.