Table_3.DOCX

Plant-type ferredoxins in Arabidopsis transfer electrons from the photosystem I to multiple redox-driven enzymes involved in the assimilation of carbon, nitrogen, and sulfur. Leaf-type ferredoxins also modulate the switch between the linear and cyclic electron routes of the photosystems. Recently, two novel ferredoxin homologs with extra C-termini were identified in the Arabidopsis genome (AtFdC1, AT4G14890; AtFdC2, AT1G32550). FdC1 was considered as an alternative electron acceptor of PSI under extreme ferredoxin-deficient conditions. Here, we showed that FdC1 could interact with some, but not all, electron acceptors of leaf-type Fds, including the ferredoxin-thioredoxin reductase (FTR), sulfite reductase (SiR), and nitrite reductase (NiR). Photoreduction assay on cytochrome c and enzyme assays confirmed its capability to receive electrons from PSI and donate electrons to the Fd-dependent SiR and NiR but not to the ferredoxin-NADP+ oxidoreductase (FNR). Hence, FdC1 and leaf-type Fds may play differential roles by channeling electrons from photosystem I to different downstream electron acceptors in photosynthetic tissues. In addition, the median redox potential of FdC1 may allow it to receive electrons from FNR in non-photosynthetic plastids.

拟南芥（Arabidopsis）中的植物型铁氧还蛋白（ferredoxin）可将电子从光系统I（photosystem I，PSI）传递至参与碳、氮、硫同化的多种氧化还原驱动酶类。叶型铁氧还蛋白还可调控光合系统的线性电子传递与循环电子传递途径之间的切换。近期，研究者在拟南芥基因组中鉴定出两种带有额外C端结构域的新型铁氧还蛋白同源物：AtFdC1（AT4G14890）与AtFdC2（AT1G32550）。此前认为，在极端铁氧还蛋白缺失条件下，FdC1可作为PSI的替代性电子受体。本研究证实，FdC1可与部分（而非全部）叶型铁氧还蛋白的电子受体相互作用，包括铁氧还蛋白-硫氧还蛋白还原酶（ferredoxin-thioredoxin reductase，FTR）、亚硫酸还原酶（sulfite reductase，SiR）以及亚硝酸还原酶（nitrite reductase，NiR）。细胞色素c光还原实验与酶活测定结果证实，其能够接收来自PSI的电子，并将电子传递至依赖铁氧还蛋白的SiR与NiR，但无法传递给铁氧还蛋白-NADP+氧化还原酶（ferredoxin-NADP+ oxidoreductase，FNR）。由此可见，FdC1与叶型铁氧还蛋白可能通过将电子从光系统I引导至光合组织中的不同下游电子受体，从而发挥差异化的生物学功能。此外，FdC1的中位氧化还原电位使其能够在非光合质体中接收来自FNR的电子。