State Key Laboratory of Agricultural Microbiology, College of Life Science and Technology, Huazhong Agricultural University, 430070 Wuhan, China.
Department of Molecular Sciences, Macquarie University, Sydney, NSW 2109, Australia.
Proc Natl Acad Sci U S A. 2021 May 18;118(20). doi: 10.1073/pnas.2104443118.
Biosyntheses of chlorophyll and heme in oxygenic phototrophs share a common trunk pathway that diverges with insertion of magnesium or iron into the last common intermediate, protoporphyrin IX. Since both tetrapyrroles are pro-oxidants, it is essential that their metabolism is tightly regulated. Here, we establish that heme-derived linear tetrapyrroles (bilins) function to stimulate the enzymatic activity of magnesium chelatase (MgCh) via their interaction with GENOMES UNCOUPLED 4 (GUN4) in the model green alga A key tetrapyrrole-binding component of MgCh found in all oxygenic photosynthetic species, GUN4, also stabilizes the bilin-dependent accumulation of protoporphyrin IX-binding CHLH1 subunit of MgCh in light-grown cells by preventing its photooxidative inactivation. Exogenous application of biliverdin IXα reverses the loss of CHLH1 in the bilin-deficient heme oxygenase () mutant, but not in the mutant. We propose that these dual regulatory roles of GUN4:bilin complexes are responsible for the retention of bilin biosynthesis in all photosynthetic eukaryotes, which sustains chlorophyll biosynthesis in an illuminated oxic environment.
在产氧光合作用生物中，叶绿素和血红素的生物合成共享一条共同的主干途径，该途径在最后一个共同中间体原卟啉 IX 中插入镁或铁后发生分歧。由于这两种四吡咯都是促氧化剂，因此必须对它们的代谢进行严格调控。在这里，我们确定血红素衍生的直链四吡咯（胆红素）通过与模型绿藻中的基因组不偶联 4（GUN4）相互作用来刺激镁螯合酶（MgCh）的酶促活性。在所有产氧光合作用物种中都发现的 MgCh 的关键四吡咯结合组件 GUN4，还通过防止其光氧化失活来稳定光生长细胞中依赖于胆红素的 MgCh 原卟啉 IX 结合 CHLH1 亚基的积累。外源性胆红素 IXα 的应用可逆转胆红素缺陷的血红素加氧酶（）突变体中 CHLH1 的损失，但不能在突变体中逆转。我们提出，GUN4：胆红素复合物的这两个双重调节作用负责在所有光合真核生物中保留胆红素生物合成，这维持了在光照好氧环境中的叶绿素生物合成。
