Max-Planck-Institut für Molekulare Pflanzenphysiologie, Am Mühlenberg 1, D-14476 Potsdam, Germany.
Plant Physiol. 2021 Apr 2;185(3):1111-1130. doi: 10.1093/plphys/kiaa052.
The pathway of photosystem II (PSII) assembly is well understood, and multiple auxiliary proteins supporting it have been identified, but little is known about rate-limiting steps controlling PSII biogenesis. In the cyanobacterium Synechocystis PCC6803 and the green alga Chlamydomonas reinhardtii, indications exist that the biosynthesis of the chloroplast-encoded D2 reaction center subunit (PsbD) limits PSII accumulation. To determine the importance of D2 synthesis for PSII accumulation in vascular plants and elucidate the contributions of transcriptional and translational regulation, we modified the 5'-untranslated region of psbD via chloroplast transformation in tobacco (Nicotiana tabacum). A drastic reduction in psbD mRNA abundance resulted in a strong decrease in PSII content, impaired photosynthetic electron transport, and retarded growth under autotrophic conditions. Overexpression of the psbD mRNA also increased transcript abundance of psbC (the CP43 inner antenna protein), which is co-transcribed with psbD. Because translation efficiency remained unaltered, translation output of pbsD and psbC increased with mRNA abundance. However, this did not result in increased PSII accumulation. The introduction of point mutations into the Shine-Dalgarno-like sequence or start codon of psbD decreased translation efficiency without causing pronounced effects on PSII accumulation and function. These data show that neither transcription nor translation of psbD and psbC are rate-limiting for PSII biogenesis in vascular plants and that PSII assembly and accumulation in tobacco are controlled by different mechanisms than in cyanobacteria or in C. reinhardtii.
光合作用系统 II（PSII）的组装途径已经得到很好的理解，并且已经鉴定出多种支持它的辅助蛋白，但对于控制 PSII 生物发生的限速步骤知之甚少。在蓝藻集胞藻 PCC6803 和绿藻莱茵衣藻中，存在着叶绿体编码的 D2 反应中心亚基（PsbD）生物合成限制 PSII 积累的迹象。为了确定 D2 合成对维管束植物 PSII 积累的重要性，并阐明转录和翻译调控的贡献，我们通过叶绿体转化在烟草（Nicotiana tabacum）中修饰了 psbD 的 5'非翻译区。psbD mRNA 丰度的急剧降低导致 PSII 含量显著减少，光合电子传递受损，并在自养条件下生长迟缓。psbD mRNA 的过表达也增加了 psbC（CP43 内部天线蛋白）的转录物丰度，psbC 与 psbD 共转录。由于翻译效率保持不变，psbD 和 psbC 的翻译产物随着 mRNA 丰度的增加而增加。然而，这并没有导致 PSII 积累的增加。将点突变引入到 Shine-Dalgarno 样序列或 psbD 的起始密码子中，降低了翻译效率，而对 PSII 积累和功能没有明显影响。这些数据表明，在维管束植物中，psbD 和 psbC 的转录和翻译都不是 PSII 生物发生的限速步骤，并且烟草中的 PSII 组装和积累受到与蓝藻或莱茵衣藻不同的机制控制。
