Abstract
Cyclization of acyclic lycopene by cyclases marks an important regulatory point in carotenoid biosynthesis. Though some algal lycopene epsilon cyclases (LCYEs) have been predicted computationally, very few have been functionally identified. Little is known about the regulation mechanisms of algal LCYEs. Recent comparative genomic analysis suggested that Haematococcus pluvialis contained only the β type cyclase (HpLCYB). However, in this study, carotenoid profiling found trace α-carotene in the salt-treated cells, indicating the in vivo activity of HpLCYE, a missing component for α-branch carotenoids. Thus, genes coding for HpLCYB and HpLCYE were isolated and functionally complemented in Escherichia coli. Substrate specificity assays revealed an exclusive cyclization order of HpLCYE to HpLCYB for the biosynthesis of heterocyclic carotenoids. Expression pattern studies and bioinformatic analysis of promoter regions showed that both cyclases were differentially regulated by the regulatory cis-acting elements in promoters to correlate with primary and secondary carotenoid biosynthesis under environmental stresses. Characterization of the branch components in algal carotenoid biosynthesis revealed a mechanism for control of metabolic flux into α- and β-branch by the competition and cooperation between HpLCYE and HpLCYB; and supplied a promising route for molecular breeding of cyclic carotenoid biosynthesis.
Highlights
Carotenoids are essential components in all photosynthetic apparatus of plants, algae, and cyanobacteria, fulfilling crucial functions such as photoprotection as antioxidant by quenching of triplet chlorophylls and scavenging of various reactive oxygen species (ROS), and photosynthesis as accessory light-harvesting pigments in the photosynthetic antenna complexes (Lao et al, 2011)
Carotenoid Profiles of H. pluvialis under Environmental Stresses Indicate the In Vivo Activity of a ε Cyclase
The content of astaxanthin between the control and nutrient starvation did not vary significantly. This unconspicuous difference is likely to result from the prolonged cultivation period (30 days) used, which was indicated by the similarity in morphology, i.e., partly mature cysts developed in the control (Figure 2A), N deficiency (Figure 2D), and P deficiency (Figure 2E)
Summary
Carotenoids are essential components in all photosynthetic apparatus of plants, algae, and cyanobacteria, fulfilling crucial functions such as photoprotection as antioxidant by quenching of triplet chlorophylls and scavenging of various reactive oxygen species (ROS), and photosynthesis as accessory light-harvesting pigments in the photosynthetic antenna complexes (Lao et al, 2011). Natural carotenoids can be chemically divided into carotenes and xanthophylls according to whether they contain oxygen. Xanthophylls are oxygenated molecules with oxygen being present as hydroxyl groups (e.g., lutein) or as oxo-groups (e.g., canthanxanthin) or as a combination of both groups (e.g., astaxanthin) (Jackson et al, 2008). HpLCYE for Carotenoid Cyclization begins with sequential synthesis of a series of acyclic carotenes, e.g., phytoene, ζ-carotene, neurosporene, and lycopene, from the condensation of two geranylgeranyl diphosphate (GGPP) molecules (Figure 1). These steps constitute the most common part of carotenoid biosynthesis in photosynthetic organisms
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