Park Y I, Sandström S, Gustafsson P, Oquist G
Department of Biology, Chungnam National University, Taejón, Korea.
Mol Microbiol. 1999 Apr;32(1):123-9. doi: 10.1046/j.1365-2958.1999.01332.x.
Iron deficiency is known to suppress primary productivity in both marine and freshwater ecosystems. In response to iron deficiency, certain cyanobacteria induce a chlorophyll (Chl)-protein complex, CP43', which is encoded by the isiA gene. The deduced amino-acid sequence of CP43' predicts some structural similarity to the CP43 polypeptide of photosystem II, but the function of CP43' remains uncertain. In order to assess its physiological role, the isiA gene of a cyanobacterium, Synechococcus sp. PCC7942, was inactivated by insertion mutagenesis (giving isiA cells). Compared with isiA cells, under iron deprivation, wild-type cells showed both lower rates of photosystem II-mediated O2 evolution at limiting light irradiances and decreased yields of room temperature Chl fluorescence at various irradiances. These observations strongly suggest that the decreased photosystem II activity in wild-type cells with CP43' is attributable to increased non-radiative dissipation of light energy. In agreement with this hypothesis, isiA cells were more susceptible to photoinhibition of photosynthesis than wild-type cells, resulting in much slower growth rates under iron limitation. Based on these results, we suggest that CP43' functions as a non-radiative dissipator of light energy, thus protecting photosystem II from excessive excitation under iron-deficient conditions.
众所周知，缺铁会抑制海洋和淡水生态系统的初级生产力。作为对缺铁的响应，某些蓝细菌会诱导一种由isiA基因编码的叶绿素（Chl）-蛋白复合物CP43'。CP43'推导的氨基酸序列预测其与光系统II的CP43多肽存在一些结构相似性，但CP43'的功能仍不确定。为了评估其生理作用，通过插入诱变使蓝细菌聚球藻属PCC7942的isiA基因失活（得到isiA细胞）。与isiA细胞相比，在缺铁条件下，野生型细胞在低光强下光系统II介导的O2释放速率较低，并且在各种光强下室温Chl荧光产量均降低。这些观察结果强烈表明，具有CP43'的野生型细胞中光系统II活性降低归因于光能非辐射耗散增加。与该假设一致，isiA细胞比野生型细胞更容易受到光合作用光抑制的影响，导致在铁限制条件下生长速率慢得多。基于这些结果，我们认为CP43'作为光能的非辐射耗散体发挥作用，从而在缺铁条件下保护光系统II免受过度激发。
