數千種由細胞核基因所解碼合成的蛋白質必須被準確地送入色質體中，才能促進色質體正常的生合成。而位於葉綠體外膜及內膜上的轉運蛋白複合體（Toc及Tic）則負責辨識及運輸這些色質體前軀蛋白質。在前人的豌豆研究中，Toc159蛋白質被鑑定是色質體前軀蛋白主要的辨識受體。阿拉伯芥中含有四種豌豆psToc159的同源蛋白，分別稱為atToc159、atToc132、atToc120和atToc90。這些At TOC159家族成員的基因必須被適當的調節，因為這些運輸蛋白機組必須準確的組裝以執行適當的辨識及運輸功能。為了解At TOC159家族成員的基因調節機制，這些基因的上游調節序列接上GUS報導基因後轉殖進入野生型阿拉伯芥植株。根據GUS活性分析的結果，顯示這些基因的表現模式不同。一般而言，At TOC159、At TOC132/90和At TOC120基因在不同的發育階段與不同組織中分別有高量、中度和低量的表現。在根的組織中，At TOC159及At TOC132有較高表現量，但At TOC120及At TOC90只維持基本表現量。此外，四個家族成員基因在花及果實發育的過程中都維持一定的高表現量，At TOC159在果實發育後期表現量的提高尤其顯著。此外，光處理及領導內插子的存在與否，會影響At TOC159家族成員的組織專一性表現。光訊號會促進At TOC159及At TOC90在綠色組織中的表現量，且明顯促進At TOC120在下胚軸和根組織中的表現量。領導內插子會增加At TOC120在根/葉/花/果莢組織中的表現量，但是增加At TOC90在葉/花葯組織中的表現量。除了進行不同組織的基因表現之比較分析外，我們也進行了這些轉殖株中基因的細胞專一性表現的研究。令人驚訝的，在與At TOC159和At TOC132的比較下，At TOC90和At TOC120在子葉保衛細胞中基因的表現量明顯地要比葉肉細胞多，這表示atToc90和atToc120在保衛細胞運輸蛋白質進入葉綠體上可能扮演一定的角色。這些結果顯示阿拉伯芥四種TOC159基因成員在色質體發育過程中，因為在不同組織中的色質體內所需要蛋白種類不同，所以他們必須受到不同的機制調節而顯現出基因表現上的差異，進而辨識及運輸特定前軀蛋白以維持色質體功能及發育。

Accurate import of thousands of nuclear-encoded proteins is an important step in plastid biogenesis. The import machinery of cytosolic precursor proteins to plastids relies on the Toc and Tic (translocons on the outer envelope and inner envelope membrane of chloroplasts) complexes. Toc159 protein was identified in pea (Pisum sativum) as a major receptor for the precursor proteins. In Arabidopsis thaliana, four psToc159 homologs are identified, termed atToc159, atToc132, atToc120 and atToc90. The expression of At TOC159, At TOC132, At TOC120 and At TOC90 genes have to be properly regulated, because their gene products must be correctly integrated to appropriate apparatus to perform their functions. In order to elucidate the regulatory mechanisms of At TOC159 homologous gene expression, transgenes containing various lengths of the upstream regulatory sequences of At TOC159/ At TOC132/ At TOC120/ At TOC90 and GUS coding sequence were transferred to wild type Arabidopsis. On the basis of the analysis of GUS activity in these transgenic plants, At TOC159/ At TOC132/ At TOC120/ At TOC90 had distinct expression patterns. In general, At TOC159, At TOC132/ At TOC90, and At TOC120 have relatively high, moderate, and low expression levels in various tissues of different developmental stages. In mature roots, At TOC159 and At TOC132 are expressed at higher levels, but At TOC120 and At TOC90 are expressed at the basal level. All four genes have increased expression level during flower and fruit development, particularly a remarkably high expression level of At TOC159 in later stage of fruit development. Furthermore, light treatment and leader intron in the 5' UTR induce the expression level of At TOC159 members in a tissue-specific manner. Light signal increases the At TOC159 and At TOC90 expression in green tissues, and At TOC120 expression in hypocotyls and roots. The endogenous leader intron sequence is able to up-regulate the At TOC120 expression in roots/leaves/flowers/siliques, and the At TOC90 expression in leaves/anthers. In addition to tissue-specific expression, the cell-specific expression of these transgenic plants was also determined. Surprisedly, At TOC90 and At TOC120 had higher GUS activity in the guard cells than mesophyll cells of cotyledon when compared to At TOC159 and At TOC132. This indicates that atToc90 and atToc120 might play a more important role in regulating chloroplast protein import of guard cells. These results suggest that differential expression of At TOC159 gene members is essential during plastid development, because proper atToc159 isoforms are required to import distinct proteins to the plastids of different tissues.

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