title: On the substrate specificity of the rice strigolactone biosynthesis enzyme DWARF27.
abstract: The β-carotene isomerase OsDWARF27 is stereo- and double bond-specific. It converts bicyclic carotenoids with at least one unsubstituted β-ionone ring. OsDWARF27 may contribute to the formation of α-carotene-based strigolactone-like compounds. Strigolactones (SLs) are synthesized from all-trans-β-carotene via a pathway involving the β-carotene isomerase DWARF27, the carotenoid cleavage dioxygenases 7 and 8 (CCD7, CCD8), and cytochrome P450 enzymes from the 711 clade (MAX1 in Arabidopsis). The rice enzyme DWARF27 was shown to catalyze the reversible isomerization of all-trans- into 9-cis-β-carotene in vitro. β-carotene occurs in different cis-isomeric forms, and plants accumulate other carotenoids, which may be substrates of DWARF27. Here, we investigated the stereo and substrate specificity of the rice enzyme DWARF27 in carotenoid-accumulating E. coli strains and in in vitro assays performed with heterologously expressed and purified enzyme. Our results suggest that OsDWARF27 is strictly double bond-specific, solely targeting the C9-C10 double bond. OsDWARF27 did not introduce a 9-cis-double bond in 13-cis- or 15-cis-β-carotene. Substrates isomerized by OsDWARF27 are bicyclic carotenoids, including β-, α-carotene and β,β-cryptoxanthin, that contain at least one unsubstituted β-ionone ring. Accordingly, OsDWARF27 did not produce the abscisic acid precursors 9-cis-violaxanthin or -neoxanthin from the corresponding all-trans-isomers, excluding a direct role in the formation of this carotenoid derived hormone. The conversion of all-trans-α-carotene yielded two different isomers, including 9'-cis-α-carotene that might be the precursor of strigolactones with an ε-ionone ring, such as the recently identified heliolactone.
authors.affiliation: Faculty of Biology, Albert-Ludwigs University of Freiburg, Schaenzlestr. 1, 79104, Freiburg, Germany.
authors.forename: Mark
authors.initials: M
authors.lastname: Bruno
authors.affiliation: BESE Division, King Abdullah University of Science and Technology (KAUST), 4700, 23955-6900, Thuwal, Kingdom of Saudi Arabia. salim.babili@kaust.edu.sa.|Faculty of Biology, Albert-Ludwigs University of Freiburg, Schaenzlestr. 1, 79104, Freiburg, Germany. salim.babili@kaust.edu.sa.
authors.forename: Salim
authors.initials: S
authors.lastname: Al-Babili
mesh_terms: D053898:Biosynthetic Pathways; D002338:Carotenoids / Q000737:chemistry / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D007535:Isomerases / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D007783:Lactones / Q000378:metabolism*; D012275:Oryza / Q000378:metabolism*; D010937:Plant Growth Regulators / Q000378:metabolism*; D010940:Plant Proteins / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D013379:Substrate Specificity
chemical_list: D007783:Lactones; D010937:Plant Growth Regulators; D010940:Plant Proteins; D002338:Carotenoids; D007535:Isomerases
keywords: Apocarotenoids; Carlactone; Carotene isomerase; Carotenoid cleavage dioxygenase; Carotenoids; Strigolactones