Institute of Low Temperature Science, Hokkaido University, N19 W8 Kita-Ku, Japan.
J Biol Chem. 2013 Jul 5;288(27):19330-41. doi: 10.1074/jbc.M113.462663. Epub 2013 May 15.
Chlorophyll b is found in photosynthetic prokaryotes and primary and secondary endosymbionts, although their light-harvesting systems are quite different. Chlorophyll b is synthesized from chlorophyll a by chlorophyllide a oxygenase (CAO), which is a Rieske-mononuclear iron oxygenase. Comparison of the amino acid sequences of CAO among photosynthetic organisms elucidated changes in the domain structures of CAO during evolution. However, the evolutionary relationship between the light-harvesting system and the domain structure of CAO remains unclear. To elucidate this relationship, we investigated the CAO structure and the pigment composition of chlorophyll-protein complexes in the prasinophyte Micromonas. The Micromonas CAO is composed of two genes, MpCAO1 and MpCAO2, that possess Rieske and mononuclear iron-binding motifs, respectively. Only when both genes were introduced into the chlorophyll b-less Arabidopsis mutant (ch1-1) was chlorophyll b accumulated, indicating that cooperation between the two subunits is required to synthesize chlorophyll b. Although Micromonas has a characteristic light-harvesting system in which chlorophyll b is incorporated into the core antennas of reaction centers, chlorophyll b was also incorporated into the core antennas of reaction centers of the Arabidopsis transformants that contained the two Micromonas CAO proteins. Based on these results, we discuss the evolutionary relationship between the structures of CAO and light-harvesting systems.
叶绿素 b 存在于光合原核生物和初级和次级内共生体中，尽管它们的光收集系统有很大的不同。叶绿素 b 由叶绿素 a 通过叶绿素 a 加氧酶（CAO）合成，CAO 是一种 Rieske-单核铁氧合酶。光合生物中 CAO 的氨基酸序列比较阐明了 CAO 的结构域在进化过程中的变化。然而，光收集系统和 CAO 的结构域结构之间的进化关系尚不清楚。为了阐明这种关系，我们研究了绿藻门中前绿藻的 CAO 结构和叶绿素蛋白复合物的色素组成。Micromonas 的 CAO 由两个基因组成，即 MpCAO1 和 MpCAO2，它们分别具有 Rieske 和单核铁结合基序。只有当这两个基因被引入叶绿素 b 缺失的拟南芥突变体（ch1-1）中时，才会积累叶绿素 b，表明两个亚基之间的合作是合成叶绿素 b 所必需的。尽管 Micromonas 具有一种特征性的光收集系统，其中叶绿素 b 被整合到反应中心的核心天线中，但叶绿素 b 也被整合到含有两种 Micromonas CAO 蛋白的拟南芥转化体的反应中心核心天线中。基于这些结果，我们讨论了 CAO 和光收集系统结构之间的进化关系。
