Characterization of a β-carotene isomerase from the cyanobacterium .

Carotenoids are essential components of the photosynthetic apparatus and precursors of plant hormones, such as strigolactones (SLs). SLs are involved in various aspects of plant development and stress-response processes, including the establishment of root and shoot architecture. SL biosynthesis begins with the reversible isomerization of all--carotene into 9--β-carotene, catalysed by DWARF27 β-carotene isomerase (D27). Sequence comparisons have revealed the presence of D27-related proteins in photosynthetic eukaryotes and cyanobacteria lacking SLs. To gain insight into the evolution of SL biosynthesis, we characterized the activity of a cyanobacterial D27 protein (D27) from , using carotenoid-accumulating cells and enzymatic assays. Our results demonstrate that D27 is an all-/ and /-β-carotene isomerase, with a / conversion preference. D27 catalysed 13-/15-, all-/9--β-carotene, and neurosporene isomerization. Compared with plant enzymes, it exhibited a lower 9--/all--β-carotene conversion ratio. A comprehensive genome survey revealed the presence of as a single-copy gene in the genomes of 20 out of 200 cyanobacteria species analysed. Phylogenetic and enzymatic analysis of D27 indicated that cyanobacterial genes form a single orthologous group, which is considered an ancestral type of those found in photosynthetic eukaryotes. This article is part of the theme issue 'The evolution of plant meta‌bolism'.