Tobacco rbcL deletion mutant, which lacks the key enzyme Rubisco for photosynthetic carbon assimilation, was characterized with respect to thylakoid functional properties and protein composition. The Delta rbcL plants showed an enhanced capacity for dissipation of light energy by non-photochemical quenching which was accompanied by low photochemical quenching and low overall photosynthetic electron transport rate. Flash-induced fluorescence relaxation and thermoluminescence measurements revealed a slow electron transfer and decreased redox gap between Q(A) and Q(B), whereas the donor side function of the Photosystem II (PSII) complex was not affected. The 77 K fluorescence emission spectrum of Delta rbcL plant thylakoids implied a presence of free light harvesting complexes. Mutant plants also had a low amount of photooxidisible P700 and an increased ratio of PSII to Photosystem I (PSI). On the other hand, an elevated level of plastid terminal oxidase and the lack of F0 'dark rise' in fluorescence measurements suggest an enhanced plastid terminal oxidase-mediated electron flow to O2 in Delta rbcL thylakoids. Modified electron transfer routes together with flexible dissipation of excitation energy through PSII probably have a crucial role in protection of PSI from irreversible protein damage in the Delta rbcL mutant under growth conditions. This protective capacity was rapidly exceeded in Delta rbcL mutant when the light level was elevated resulting in severe degradation of PSI complexes.

译文

烟草rbcL缺失突变体缺乏光合碳同化的关键酶Rubisco,其特征在于类囊体功能特性和蛋白质组成。Delta rbcL植物通过非光化学猝灭显示出增强的光能耗散能力,伴随着低光化学猝灭和低的总体光合电子传输速率。闪光诱导的荧光弛豫和热释光测量显示,电子转移缓慢,Q (a) 和Q(B) 之间的氧化还原间隙减小,而光系统II (PSII) 复合物的供体侧功能不受影响。Δ rbcL植物类囊体的77 k荧光发射光谱暗示存在自由的光收集复合物。突变植物的光氧化P700含量低,PSII与光系统I (PSI) 的比率增加。另一方面,荧光测量中质体末端氧化酶的水平升高和F0 “暗升” 的缺乏表明,在 δ rbcL类囊体中,质体末端氧化酶介导的向O2的电子流增强。修饰的电子转移途径以及通过PSII的激发能的灵活耗散可能在保护PSI免受生长条件下Delta rbcL突变体的不可逆蛋白质损伤方面起着至关重要的作用。当光照水平升高导致PSI复合物严重降解时,Delta rbcL突变体迅速超过了这种保护能力。

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