The Biosynthetic Pathway for Synechoxanthin, an Aromatic Carotenoid Synthesized by the Euryhaline, Unicellular Cyanobacterium Synechococcus sp. Strain PCC 7002

Abstract

The euryhaline, unicellular cyanobacterium Synechococcus sp. strain PCC 7002 produces the dicyclic aromatic carotenoid synechoxanthin (χ,χ-caroten-18,18′-dioic acid) as a major pigment (>15% of total carotenoid) and when grown to stationary phase also accumulates small amounts of renierapurpurin (χ,χ-carotene) (J. E. Graham, J. T. J. Lecomte, and D. A. Bryant, J. Nat. Prod. 71:1647-1650, 2008). Two genes that were predicted to encode enzymes involved in the biosynthesis of synechoxanthin were identified by comparative genomics, and these genes were insertionally inactivated in Synechococcus sp. strain PCC 7002 to verify their function. The cruE gene (SYNPCC7002_A1248) encodes β-carotene desaturase/methyltransferase, which converts β-carotene to renierapurpurin. The cruH gene (SYNPCC7002_A2246) encodes an enzyme that is minimally responsible for the hydroxylation/oxidation of the C-18 and C-18′ methyl groups of renierapurpurin. Based on observed and biochemically characterized intermediates, a complete pathway for synechoxanthin biosynthesis is proposed.

The Biosynthetic Pathway for Myxol-2′ Fucoside (Myxoxanthophyll) in the Cyanobacterium Synechococcus sp. Strain PCC 7002

“…strain PCC 6803 ORF sll0254 serves as a bifunctional lycopene cyclase/dioxygenase responsible for -end oxygenations in myxoxanthophyll biosynthesis. This conclusion has previously been shown to be incorrect (12,26). sll0254 instead encodes an enzyme, designated CruE, which is responsible for the conversion of ␤-rings to -rings during the synthesis of the aromatic xanthophyll synechoxanthin (12,13).…”

“…This conclusion has previously been shown to be incorrect (12,26). sll0254 instead encodes an enzyme, designated CruE, which is responsible for the conversion of ␤-rings to -rings during the synthesis of the aromatic xanthophyll synechoxanthin (12,13). Because all previous studies failed to identify genes uniquely required for myxoxanthophyll biosynthesis, we sought to identify the genes that encode the enzymes responsible for the two -end hydroxylations and the glycosylation that must occur during myxoxanthophyll biosynthesis.…”

“…Compound mx was purified from the cruG mutant by preparative HPLC, and hydroxyl-counting experiments were conducted by treating samples with Sigma-Sil-A reagent as described previously (5,12). Compound mx could be modified up to three times by trimethylsilylation, which verified that three hydroxyl groups were present (see Fig.…”

“…strain PCC 6803 or Synechococcus sp. strain PCC 7002 (12,26). Instead, CruE is a carotene desaturase/methyltransferase that converts ␤-rings into -rings during the synthesis of synechoxanthin (12).…”

“…strain PCC 6803 (31). However, subsequent studies have shown that the sll0254 product and its orthologs, renamed CruE, are carotene desaturases/methyltransferases that participate in the synthesis of the aromatic carotenoid synechoxanthin (12,13).…”