A neighboring aromatic-aromatic amino acid combination governs activity divergence of tomato PSY1 and PSY2

Authors: CAO, HONGBO - Cornell University; LUO, HONGMEI - Cornell University; YUAN, HUI - Cornell University; EISSA, MOHAMED - Cornell University; Thannhauser, Theodore - Ted; WELSCH, RALF - University Of Freiburg; HAO, YUJIN - Shandong Agricultural University; CHENG, LAIIANG - Cornell University; Li, Li

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/2019
Publication Date: 6/20/2020
Citation: Cao, H., Luo, H., Yuan, H., Eissa, M.A., Thannhauser, T.W., Welsch, R., Hao, Y., Cheng, L., Li, L. 2020. A neighboring aromatic-aromatic amino acid combination governs activity divergence of tomato PSY1 and PSY2. Plant Physiology. 180(4):1988-2003. https://doi.org/10.1104/pp.19.00384.
DOI: https://doi.org/10.1104/pp.19.00384

Interpretive Summary: Carotenoids are indispensable to plants and humans. Phytoene synthase (PSY), as the major rate-limiting enzyme for carotenoid biosynthesis, is normally found as a small enzyme family in crops. However, limited information is available for PSY isoforms in relation to their respective enzyme activities and for the amino acid residues critical for PSY activity. In this study, we found that tomato fruit-specific PSY1 unexpectedly is less effective in promoting carotenogenesis than leaf-specific PSY2. We identified the key amino acid residues responsible for the activity divergence. Moreover, we experimentally confirmed the critical role of an aromatic residue in the PSY active site for its activity, and elucidated the necessity of one neighboring aromatic-aromatic combination for high PSY activity. Furthermore, we examined the evolutionary features of these key residues toward high PSY activity. Our findings provide new insights into the functional divergence of PSY isoforms and highlight the potential to rationally design PSY for targeted metabolic engineering or breeding of carotenoid-enriched crops.

Technical Abstract: Phytoene synthase (PSY) is a major rate-limiting enzyme of carotenogenesis. PSY is normally found as a small enzyme family with up to three members in plants. However, knowledge of PSY isoforms in relation to their respective enzyme activities and amino acid residues important for PSY activity is limited. In this study, we focused on two tomato PSY isoforms, PSY1 and PSY2, and investigated their abilities to catalyze carotenogenesis via heterologous expression in transgenic Arabidopsis and bacterial systems. Interestingly, we found that the fruit-specific PSY1 is less effective in promoting carotenoid biosynthesis than the green tissue-specific PSY2. Examination of the basis by site-directed mutagenesis analysis and 3D structure modeling of the PSY proteins revealed two key amino acid residues responsible for PSY1 and PSY2 activity divergence, identifying a neighboring aromatic-aromatic combination in one of the PSY core structures being crucial for high PSY activity. Remarkably, this neighboring aromatic-aromatic combination is evolutionarily conserved among plant PSYs except PSY1 of tomato and potato. A strong transcription of PSY1 likely evolved to compensate its weak enzyme activity for massive carotenoid biosynthesis in ripe tomato fruit. This study provides new insights into the functional divergence of PSY isoforms and highlights the potential to rationally design PSY for effective development of carotenoid enriched crops.