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 Chlamydomonas reinhardtii. A key tetrapyrrole-binding component of MgCh found in all oxygenic photosynthetic species, CrGUN4, also stabilizes the bilin-dependent accumulation of protoporphyrin IX-binding CrCHLH1 subunit of MgCh in light-grown C. reinhardtii cells by preventing its photooxidative inactivation. Exogenous application of biliverdin IXα reverses the loss of CrCHLH1 in the bilin-deficient heme oxygenase (hmox1) mutant, but not in the gun4 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.

含氧光养生物中叶绿素和血红素的生物合成共享一条共同的主干通路，该通路随着镁或铁插入最后一个共同的中间体原卟啉 IX 而发生分歧。由于这两种四吡咯都是促氧化剂，因此必须严格调节它们的代谢。在这里，我们确定血红素衍生的线性四吡咯 (bilins) 通过与模型绿藻莱茵衣藻中的 GENOMES UNCOUPLED 4 (GUN4) 相互作用来刺激镁螯合酶 (MgCh) 的酶活性。在所有含氧光合物种Cr GUN4 中发现的 MgCh 的关键四吡咯结合成分也稳定了光生长中 MgCh的原卟啉 IX 结合Cr CHLH1 亚基的胆素依赖性积累C. reinhardtii细胞通过防止其光氧化失活。胆绿素 IXα 的外源应用逆转了胆素缺陷型血红素加氧酶 ( hmox1 ) 突变体中Cr CHLH1的丢失，但在gun4突变体中没有逆转。我们建议 GUN4:bilin 复合物的这些双重调节作用负责在所有光合真核生物中保留胆素生物合成，这在光照有氧环境中维持叶绿素生物合成。