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 β-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 β-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.