Mäenpää P, Sippola K, Rokka A, Aro E M
Department of Biology, University of Turku, Finland.
Plant Mol Biol. 1998 Dec;38(6):1191-200. doi: 10.1023/a:1006051615081.
Ala-251 in the membrane-parallel helix in the D-E loop of the D1 polypeptide close to the Q(B) pocket of photosystem II (PS II), was mutated to aspartate (D), lysine (K), leucine (L) or serine (S) in Synechocystis 6803. O2 evolution rates (H2O-->DCBQ; 2,6-dichloro-p-benzoquinone) of A251D, A251L and A251S were lower, being 38, 16, 62 and 70%, respectively, of that of the control, and there was an even more drastic impairment of O2 evolution when measured from H2O to DMBQ (2,5-dimethyl-p-benzoquinone), demonstrating modifications in the Q(B) pocket. However, in all other mutants but A251K, the Q(B) function could sustain O2 evolution at a level high enough to support photosynthetic growth. The mutant A251S, carrying a substitution of alanine for a chemically quite similar residue serine, was less severely affected. Substitution by a positively charged residue drastically delayed chlorophyll a fluorescence relaxation in the non-photosynthetic strain A251K, implying strong impairment of Q(A)-to-Q(B) electron transfer. Delay of fluorescence relaxation was clear in A251D as well, carrying a substitution of alanine for a negatively charged residue. The effects of the substitutions of A251 demonstrate the importance of this residue of the D1 polypeptide in the conformation of the acceptor side of PS II and, accordingly, the effect on the acceptor-side function of PS II was very clear. Nevertheless, the tolerance of PS II activity to high-light-induced photoinhibition in vivo and the subsequent D1 degradation were not much impaired in any of the photosynthetic mutant strains as compared to the control.
在集胞藻6803中，靠近光系统II（PS II）的Q(B)口袋的D1多肽的D-E环中与膜平行的螺旋上的丙氨酸-251被突变为天冬氨酸（D）、赖氨酸（K）、亮氨酸（L）或丝氨酸（S）。A251D、A251L和A251S的O2释放速率（H2O→DCBQ；2,6-二氯对苯醌）较低，分别为对照的38%、16%、62%和70%，并且当从H2O到DMBQ（2,5-二甲基对苯醌）测量时，O2释放有更严重的损伤，这表明Q(B)口袋发生了改变。然而，除A251K外的所有其他突变体中，Q(B)功能能够维持足够高的O2释放水平以支持光合生长。携带化学性质非常相似的丝氨酸取代丙氨酸的突变体A251S受到的影响较小。在非光合菌株A251K中，用带正电荷的残基取代导致叶绿素a荧光弛豫大幅延迟，这意味着Q(A)到Q(B)的电子传递受到严重损伤。在携带带负电荷残基取代丙氨酸的A251D中，荧光弛豫延迟也很明显。A251的取代效应表明D1多肽的这个残基在PS II受体侧构象中的重要性，因此，对PS II受体侧功能的影响非常明显。然而，与对照相比，任何光合突变菌株中PS II活性对高光诱导的体内光抑制的耐受性以及随后的D1降解都没有受到太大损害。
