Nagata N, Satoh S, Tanaka R, Tanaka A
Institute of Low Temperature Science, Hokkaido University, and CREST, Japan Science and Technology Corporation, Kita-ku, N19 W8, 060-0819 Sapporo, Japan.
Planta. 2004 Apr;218(6):1019-25. doi: 10.1007/s00425-003-1181-6. Epub 2004 Jan 10.
Chlorophyll b is a photosynthetic antenna pigment found in prochlorophytes and chlorophytes. In chlorophytes, its biosynthesis regulates the photosynthetic antenna size. Chlorophyll b is synthesized from chlorophyll a in a two-step oxygenation reaction by chlorophyllide a oxygenase (CAO). In this study, we first identified the entire sequence of a prochlorophyte CAO gene from Prochlorothrix hollandica to compare it with those from chlorophytes, and we examined the catalytic activity of the gene product. Southern blot analysis showed that the CAO gene is presented in one copy in the P. hollandica genome. The P. hollandica CAO gene (PhCAO) has a coding capacity for 367 amino acids, which is much smaller than that of Arabidopsis thaliana (537 amino acids) and Oryza sativa (542 amino acids) CAO genes. In spite of the small size, PhCAO catalyzed the formation of chlorophyll b. By comparing these sequences, we classified the land-plant sequences into four parts: the N-terminal sequence predicted to be a transit peptide, the successive conserved sequence unique in land plants (A-domain, 134 amino acids), a less-conserved sequence (B-domain, 30 amino acids) and the C-terminal conserved sequence common in chlorophytes and prochlorophytes (C-domain, 337 to 344 amino acids). We demonstrated that the C-domain is sufficient for catalytic activity by transforming the cyanobacterium Synechocystis sp. PCC6803 with the C-domain from A. thaliana. In this paper, the role of the A-domain is discussed in relation to the formation of light-harvesting chlorophyll a/b-protein complexes in land plants.
叶绿素b是一种存在于原绿藻和绿藻中的光合天线色素。在绿藻中，其生物合成调节光合天线的大小。叶绿素b由叶绿素a通过叶绿素酸酯a加氧酶（CAO）催化的两步加氧反应合成。在本研究中，我们首先从荷兰原绿藻中鉴定出原绿藻CAO基因的完整序列，以便与绿藻中的序列进行比较，并检测了该基因产物的催化活性。Southern杂交分析表明，CAO基因在荷兰原绿藻基因组中以单拷贝形式存在。荷兰原绿藻CAO基因（PhCAO）编码367个氨基酸，比拟南芥（537个氨基酸）和水稻（542个氨基酸）的CAO基因小得多。尽管尺寸较小，但PhCAO催化了叶绿素b的形成。通过比较这些序列，我们将陆生植物序列分为四个部分：预测为转运肽的N端序列、陆生植物中独特的连续保守序列（A结构域，134个氨基酸）、保守性较低的序列（B结构域，30个氨基酸）以及绿藻和原绿藻中常见的C端保守序列（C结构域，337至344个氨基酸）。我们通过用拟南芥的C结构域转化集胞藻PCC6803，证明了C结构域足以具有催化活性。在本文中，我们讨论了A结构域在陆生植物中捕光叶绿素a/b蛋白复合物形成方面的作用。
