Bilin-dependent regulation of chlorophyll biosynthesis by GUN4

Author: Weiqing Zhang, Robert D. Willows, Rui Deng, Zheng Li, Mengqi Li, Yan Wang, Yunling Guo, Weida Shi, Qiuling Fan, Shelley S. Martin, Nathan C. Rockwell, J. Clark Lagarias and Deqiang Duanmu

PNAS. 2021 May 18. 118(20)

Abstract: Biosynthesis 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 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 retention of bilin biosynthesis in all photosynthetic eukaryotes to sustain chlorophyll biosynthesis in an illuminated oxic environment.

Full Article:https://doi.org/10.1073/pnas.2104443118