Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Saitama 338-8570, Japan.
Plant Physiol. 2024 Sep 2;196(1):621-633. doi: 10.1093/plphys/kiae323.
Photomixotrophic growth A (PmgA) is a pleiotropic regulator essential for growth under photomixotrophic and prolonged high-light (HL) conditions in the cyanobacterium Synechocystis sp. PCC 6803. The overall similarity with the antisigma factor of the bacterial partner-switching system indicates that PmgA exerts a regulatory function via phosphorylation of its target proteins. In this study, we performed an in vitro phosphorylation assay and protein-protein interaction analysis and found that PmgA interacts with 4 antisigma antagonist homologs, Ssr1600, Slr1856, Slr1859, and Slr1912, but specifically phosphorylates Ssr1600. Phenotypic analyses using the set of gene disruption and overexpression strains of pmgA and ssr1600 revealed that phosphorylation by PmgA is essential for the accumulation of Ssr1600 protein in vivo. The ssr1600-disrupted mutant showed similar phenotypes as those previously reported for the pmgA-disrupted mutant, namely, no obvious phenotype just after the shift to HL, but higher chlorophyll content, 5-aminolevulinic acid synthesis activity, and psaAB transcript levels than those in the wild type after 6 h. These findings indicate that the phosphorylated form of Ssr1600 works as the output of the partner-switching system to coordinately repress chlorophyll biosynthesis and accumulation of photosystem I during HL acclimation.
光混合营养生长 A（PmgA）是一种多功能调节剂，对于蓝藻 Synechocystis sp. PCC 6803 在光混合营养和长时间高光（HL）条件下的生长是必不可少的。与细菌伴侣切换系统的反西格玛因子的整体相似性表明，PmgA 通过其靶蛋白的磷酸化发挥调节功能。在这项研究中，我们进行了体外磷酸化测定和蛋白质-蛋白质相互作用分析，发现 PmgA 与 4 种反西格玛拮抗剂同源物 Ssr1600、Slr1856、Slr1859 和 Slr1912 相互作用，但特异性地磷酸化 Ssr1600。使用 pmgA 和 ssr1600 的基因敲除和过表达菌株的集合进行表型分析表明，PmgA 的磷酸化对于 Ssr1600 蛋白在体内的积累是必不可少的。ssr1600 缺失突变体表现出与 pmgA 缺失突变体相似的表型，即在转移到 HL 后不久没有明显的表型，但在 6 h 后，叶绿素含量、5-氨基酮戊酸合成活性和 psaAB 转录水平比野生型更高。这些发现表明，Ssr1600 的磷酸化形式作为伴侣切换系统的输出，在 HL 适应过程中协调抑制叶绿素生物合成和光系统 I 的积累。
