Critical residues for carotenoid biosynthesis by phytoene synthase (Plant Phys)

Carotenoids are diverse structures that contribute to photosynthetic light harvesting and serve as pigments, photoprotectants, and precursors for vitamin A and signalling molecules. Phytoene synthase is the first committed enzyme for carotenoid synthesis, and in tomato it is encoded by two genes, PSY1 and PSY2. When compared at similar protein levels, PSY1 expression results in much lower accumulation of carotenoids than PSY2, suggesting that it is a less efficient enzyme, although modelling studies suggest that both enzymes are structurally similar. To identify the cause of the different activities, Cao et al. identified several residues as potentially key. Through site-directed mutagenesis, they identified a pair of adjacent aromatic residues that are critical for high levels of activity. These high-activity associated residues are conserved in most other plants. Interestingly, PSY1 is the main enzyme expressed in tomato fruit, where high levels of transcription compensate for its lower relative activity.  (Summary by Mary Williams) Plant Physiol. 10.1104/pp.19.00384