GUN4 protein plays a regulatory role in tetrapyrrole biosynthesis and chloroplast‐to‐nucleus signalling in Chlamydomonas reinhardtii

The GENOMES UNCOUPLED 4 (GUN4) protein is found only in aerobic photosynthetic organisms. We investigated the role of GUN4 in metabolic activities of the Mg branch of the tetrapyrrole biosynthesis pathway and the plastid signal‐mediated changes of nuclear gene expression in Chlamydomonas reinhardtii. In light, gun4 accumulates only 40% of the wild‐type chlorophyll level. Light‐ or dark‐grown gun4 mutant accumulates high levels of protoporphyrin IX (Proto), and displays increased sensitivity to moderate light intensities. Despite the photooxidative stress, gun4 fails to downregulate mRNA levels of the tetrapyrrole biosynthesis and the photosynthesis‐associated nuclear genes (PhANGs). In contrast, upon illumination, the Proto‐accumulating and light‐sensitive chlD–1 mutant displays the expected downregulation of the same nuclear genes. Although chlD–1 and the wild type have similar GUN4 transcript levels, the GUN4 protein in chlD–1 is hardly detectable. Overexpression of GUN4 in chlD–1 modifies the downregulation of nuclear gene expression, but also increases light tolerance. Therefore, GUN4 is proposed to function in ‘shielding’ Proto, and most likely MgProto, by reducing reactivity with O₂. Furthermore, GUN4 seems to be involved in sensing elevated levels of these photoreactive tetrapyrrole intermediates, and contributing to ¹O₂‐mediated retrograde signalling, originating from chlorophyll biosynthesis.