Xu H, Vavilin D, Vermaas W
Department of Plant Biology and Center of the Study of Early Events in Photosynthesis, Arizona State University, Box 871601, Tempe, AZ 85287-1601, USA.
Proc Natl Acad Sci U S A. 2001 Nov 20;98(24):14168-73. doi: 10.1073/pnas.251530298.
An Arabidopsis thaliana chlorophyll(ide) a oxygenase gene (cao), which is responsible for chlorophyll b synthesis from chlorophyll a, was introduced and expressed in a photosystem I-less strain of the cyanobacterium Synechocystis sp. PCC 6803. In this strain, most chlorophyll is associated with the photosystem II complex. In line with observations by Satoh et al. [Satoh, S., Ikeuchi, M., Mimuro, M. & Tanaka, A. (2001) J. Biol. Chem. 276, 4293-4297], chlorophyll b was made but accounted for less than 10% of total chlorophyll. However, when lhcb encoding light-harvesting complex (LHC)II from pea was present in the same strain (lhcb(+)/cao(+)), chlorophyll b accumulated in the cell to levels exceeding those of chlorophyll a, although LHCII did not accumulate. In the lhcb(+)/cao(+) strain, the total amount of chlorophyll, the number of chlorophylls per photosystem II center, and the oxygen-evolving activity on a per-chlorophyll basis were similar to those in the photosystem I-less strain. Furthermore, the chlorophyll a/b ratio of photosystem II core particles (retaining CP47 and CP43) and of whole cells of the lhcb(+)/cao(+) strain was essentially identical, and PS II activity could be obtained efficiently by chlorophyll b excitation. These data indicate that chlorophyll b functionally substitutes for chlorophyll a in photosystem II. Therefore, the availability of chlorophylls, rather than their binding specificity, may determine which chlorophyll is incorporated at many positions of photosystem II. We propose that the transient presence of a LHCII/chlorophyll(ide) a oxygenase complex in the lhcb(+)/cao(+) strain leads to a high abundance of available chlorophyll b that is subsequently incorporated into photosystem II complexes. The apparent LHCII requirement for high chlorophyll(ide) a oxygenase activity may be instrumental to limit the occurrence of chlorophyll b in plants to LHC proteins.
将负责从叶绿素a合成叶绿素b的拟南芥叶绿素（酸）a加氧酶基因（cao）导入缺乏光系统I的集胞藻6803菌株中并进行表达。在该菌株中，大部分叶绿素与光系统II复合体相关。与Satoh等人的观察结果一致[Satoh, S., Ikeuchi, M., Mimuro, M. & Tanaka, A. (2001) J. Biol. Chem. 276, 4293 - 4297]，合成了叶绿素b，但占总叶绿素的比例不到10%。然而，当同一菌株中存在来自豌豆的编码光捕获复合体（LHC）II的lhcb时（lhcb(+)/cao(+)），叶绿素b在细胞中积累至超过叶绿素a的水平，尽管LHCII并未积累。在lhcb(+)/cao(+)菌株中，叶绿素总量、每个光系统II中心的叶绿素数量以及基于叶绿素的放氧活性与缺乏光系统I的菌株相似。此外，lhcb(+)/cao(+)菌株的光系统II核心颗粒（保留CP47和CP43）以及全细胞的叶绿素a/b比值基本相同，并且通过叶绿素b激发能够高效获得PS II活性。这些数据表明，叶绿素b在光系统II中功能性地替代了叶绿素a。因此，叶绿素的可用性而非其结合特异性，可能决定了在光系统II的许多位置掺入哪种叶绿素。我们提出，lhcb(+)/cao(+)菌株中LHCII/叶绿素（酸）a加氧酶复合体的短暂存在导致了大量可利用的叶绿素b，随后这些叶绿素b被掺入光系统II复合体中。对高叶绿素（酸）a加氧酶活性明显的LHCII需求可能有助于将植物中叶绿素b的出现限制在LHC蛋白中。
