A null lesion in the rhodopin 3,4-desaturase of Rhodospirillum rubrum unmasks a cryptic branch of the carotenoid biosynthetic pathway.
The carotenoids accumulated by a mutant Rhodospirillum rubrum ST4, containing a single Tn5 lesion in the pathway for carotenoid biosynthesis, were analyzed by HPLC, 1H NMR spectroscopy, and field desorption mass spectrometry. The main carotenoid was identified as 3,4,3',4'-tetrahydrospirilloxanthin, and the four minor carotenoids were identified as rhodopin, 3,4-dihydroanhydrorhodovibrin, 3', 4'-dihydrorhodovibrin, and 1,1'-dihydroxylycopene. The C-3,4 and C-3',4' bonds of all 5 carotenoids are saturated, and they have 11 conjugated double bonds. With the exception of rhodopin, which is a normal intermediate of the wild-type pathway, all of the carotenoids are not naturally occurring. The Tn5 lesion was assigned to rhodopin 3,4-desaturase which is proposed to catalyze dehydrogenation at both ends of the symmetrical spirilloxanthin derivative. An unexpected finding was that the enzymes following rhodopin 3,4-desaturase are still able to end-modify the 3,4-, and 3',4'-saturated precursors and that the order of methylation and hydroxylation is not obligatory. It is proposed that the observed nonnatural carotenoids can be explained by the inclusion of a cryptic branch, unmasked by the absence of rhodopin 3,4-desaturase, in the established linear pathway for spirilloxanthin biosynthesis. This is the first example of latent branching of the carotenoid biosynthesis pathway exhibited by a carotenoid mutant of a phototrophic bacterium.
Komori
M
M
Faculty of Science, Kwansei Gakuin University, Nishinomiya, Japan.
Ghosh
R
R
Takaichi
S
S
Hu
Y
Y
Mizoguchi
T
T
Koyama
Y
Y
Kuki
M
M
D002338:Carotenoids / Q000031:analogs & derivatives / Q000032:analysis / Q000096:biosynthesis* / Q000235:genetics*; D016296:Mutagenesis; D010088:Oxidoreductases / Q000235:genetics* / Q000378:metabolism; D010641:Phenotype; D012247:Rhodospirillum rubrum / Q000201:enzymology* / Q000235:genetics; D013379:Substrate Specificity; D024341:Xanthophylls / Q000031:analogs & derivatives*
D024341:Xanthophylls; C005638:hydroxyspheroidene; C005637:spheroidene; D002338:Carotenoids; C113102:rhodopin; D010088:Oxidoreductases; C022398:phytoene dehydrogenase; C055255:spirilloxanthin