Substrate specificities and availability of fucosyltransferase and beta-carotene hydroxylase for myxol 2'-fucoside synthesis in Anabaena sp. strain PCC 7120 compared with Synechocystis sp. strain PCC 6803

Abstract

To elucidate the biosynthetic pathways of carotenoids, especially myxol 2'-glycosides, in cyanobacteria, Anabaena sp. strain PCC 7120 (also known as Nostoc sp. strain PCC 7120) and Synechocystis sp. strain PCC 6803 deletion mutants lacking selected proposed carotenoid biosynthesis enzymes and GDP-fucose synthase (WcaG), which is required for myxol 2'-fucoside production, were analyzed. The carotenoids in these mutants were identified using high-performance liquid chromatography, field desorption mass spectrometry, and (1)H nuclear magnetic resonance. The wcaG (all4826) deletion mutant of Anabaena sp. strain PCC 7120 produced myxol 2'-rhamnoside and 4-ketomyxol 2'-rhamnoside as polar carotenoids instead of the myxol 2'-fucoside and 4-ketomyxol 2'-fucoside produced by the wild type. Deletion of the corresponding gene in Synechocystis sp. strain PCC 6803 (sll1213; 79% amino acid sequence identity with the Anabaena sp. strain PCC 7120 gene product) produced free myxol instead of the myxol 2'-dimethyl-fucoside produced by the wild type. Free myxol might correspond to the unknown component observed previously in the same mutant (H. E. Mohamed, A. M. L. van de Meene, R. W. Roberson, and W. F. J. Vermaas, J. Bacteriol. 187:6883-6892, 2005). These results indicate that in Anabaena sp. strain PCC 7120, but not in Synechocystis sp. strain PCC 6803, rhamnose can be substituted for fucose in myxol glycoside. The beta-carotene hydroxylase orthologue (CrtR, Alr4009) of Anabaena sp. strain PCC 7120 catalyzed the transformation of deoxymyxol and deoxymyxol 2'-fucoside to myxol and myxol 2'-fucoside, respectively, but not the beta-carotene-to-zeaxanthin reaction, whereas CrtR from Synechocystis sp. strain PCC 6803 catalyzed both reactions. Thus, the substrate specificities or substrate availabilities of both fucosyltransferase and CrtR were different in these species. The biosynthetic pathways of carotenoids in Anabaena sp. strain PCC 7120 are discussed.

FIG. 1.

Gene constructs used for generation of mutants: interruption-deletion strategy used for double-crossover mutants with mutations in (A) all4826, (B) alr4009, and (C) all5123. The target genes are indicated by black boxes. PCR frag., PCR fragment(s); Hc, HincII site.

FIG. 2.

HPLC elution profiles of pigments extracted from Anabaena sp. wild-type strain PCC 7120 (profile A) and the wcaG deletion mutant (profile B). The eluent was methanol-water (9:1, vol/vol) for the first 20 min and then 100% methanol at a rate of 2.0 ml/min. The absorbance at 475 nm (solid lines) and the absorbance at 664 nm (dashed line) are shown. The numbers in parentheses are peak numbers. KM-F, 4-ketomyxol 2′-fucoside; M-F, myxol 2′-fucoside; C, canthaxanthin; E, echinenone; β, β-carotene; Chl, chlorophyll a; KM-R, 4-ketomyxol 2′-rhamnoside; M-R, myxol 2′-rhamnoside; M, myxol; DM-R, deoxymyxol 2′-rhamnoside.

FIG. 3.

HPLC elution profiles of pigments extracted from Synechocystis sp. wild-type strain PCC 6803 (profile A) and the wcaG deletion mutant (profile B). The eluent was methanol-water (9:1, vol/vol) for the first 20 min and then 100% methanol at a rate of 2.0 ml/min. The absorbance at 475 nm (solid lines) and the absorbance at 664 nm (dashed line) are shown. The numbers in parentheses are peak numbers. M-DMF, myxol 2′-dimethyfucoside; Z, zeaxanthin; OH-E, 3′-hydroxyechinenone; DM-DMF, deoxymyxol 2′-dimethylfucoside; E, echinenone; β, β-carotene; Chl, chlorophyll a; M, myxol; DM, deoxymyxol.

FIG. 4.

Biosynthetic pathways for carotenoids and their enzymes in Anabaena sp. strain PCC 7120. See the text for details.

FIG. 5.

Structure of selected carotenoid glycosides.

FIG. 6.

HPLC elution profile of pigments extracted from the crtR deletion mutant of Anabaena sp. strain PCC 7120. The eluent was methanol-water (9:1, vol/vol) for the first 20 min and then 100% methanol at a rate of 2.0 ml/min. The absorbance at 475 nm (solid line) and the absorbance at 340 nm (dashed line) are shown. The numbers in parentheses are peak numbers. KDM-F, 4-ketodeoxymyxol 2′-fucoside; DM-F, deoxymyxol 2′-fucoside; C, canthaxanthin; E, echinenone; β, β-carotene; Chl, chlorophyll a.

FIG. 7.

Absorption spectra of two carotenoids, 4-ketodeoxymyxol 2′-fucoside (Fig. 6, peak 1) (dashed line) and deoxymyxol 2′-fucoside (Fig. 6, peak 2) (solid line), in HPLC eluent.