Slr1293 in Synechocystis sp. Strain PCC 6803 Is the C-3′,4′ Desaturase (CrtD) Involved in Myxoxanthophyll Biosynthesis

Abstract

When grown at high light intensity, more than a quarter of the total carotenoids in the unicellular cyanobacterium Synechocystis consists of myxoxanthophyll, a polar carotenoid glycoside. The biosynthetic pathway of myxoxanthophyll is unknown but is presumed to involve a number of enzymes, including a C-3,4 desaturase required to add one double bond to generate 11 conjugated double bonds in the monocyclic myxoxanthophyll. A candidate for this desaturase is Slr1293, which was identified by genome similarity searching. To determine whether Slr1293 is a desaturase recognizing neurosporene and lycopene, slr1293 was expressed in Escherichia coli strains accumulating neurosporene or lycopene. Confirming such a desaturase function for Slr1293, these E. coli strains accumulated 3,4-didehydroneurosporene and 3,4-didehydrolycopene, respectively. Indeed, deletion of slr1293 in Synechocystis provides further evidence that Slr1293 is a desaturase recognizing neurosporene: In the slr1293 deletion mutant, neurosporene was found to accumulate and was further processed to produce neurosporene glycoside. Neurosporene hereby becomes a primary candidate to be the branch point molecule between carotene and myxoxanthophyll biosynthesis in this cyanobacterium. The slr1293 gene was concluded to encode a C-3,4 desaturase that is essential for myxoxanthophyll biosynthesis, and thus it was designated as crtD. Furthermore, as Slr1293 appears to recognize neurosporene and to catalyze the first committed step on the myxoxanthophyll biosynthesis pathway, Slr1293 plays a pivotal role in directing a portion of the precursor pool for carotenoid biosynthesis toward myxoxanthophyll biosynthesis in Synechocystis sp. strain PCC 6803.

Current knowledge and recent advances in understanding metabolism of the model cyanobacteriumSynechocystissp. PCC 6803

“…Geranylgeranyl diphosphate is the substrate for carotenoid biosynthesis. Carotenoids play a key role in assembly of photosynthetic complexes [153], membrane integrity and thylakoid organization [154], and as light harvesting and photoprotective pigments. Seven carotenoids have been detected in Synechocystis: synechoxanthin, myxol-2 -dimethylfucoside (myxoxanthophyll), zeaxanthin, 3 -hydroxy-echinenone, cis-zeaxanthin, echinenone and β-carotene [155].…”

Random mutagenesis and overexpression of rhodopin-3,4-desaturase allows the production of highly conjugated carotenoids in Rhodospirillum rubrum

“…Absorption spectroscopy of hexane extracts obtained from the mutants, together with MS and HPLC analysis, indicated that unusual asymmetric carotenoids, with 14 double bonds (compared to the 13 conjugated double bonds of the wildtype spirilloxanthin) had been produced at high levels. Although these unusual carotenoids have been observed at low levels previously in other production systems [1,16,20,13], this study is the first to demonstrate the production of unsaturated carotenoids with 14 conjugated double bonds at high level in a purple bacterium. We are not aware of any other organism which produces these carotenoids under normal physiological conditions.…”

“…Furthermore, all mutant spectra show a small but significant red shift of their absorbance maxima compared to the wildtype (peak maxima (nm): wild-type: 464, 490, 522; mutants: 464-469, 493-498, 525-527). Comparison with literature data [13] suggests that the main carotenoid in the PK/DP mutants could be 3,4-didehydrolycopene (peak maxima (nm): 466, 492, 527) or derivatives thereof.…”

Proteome Mapping of a Cyanobacterium Reveals Distinct Compartment Organization and Cell-Dispersed Metabolism

“…In addition, several enzymes catalyzing carotenoid biosynthesis localized to the membranes. Carotenoids play a key role in assembly of photosynthetic complexes (Tóth et al, 2015), membrane integrity, and thylakoid organization (Mohamed et al, 2004), and as light harvesting and photoprotective pigments. Seven carotenoids have been detected in Synechocystis: synechoxanthin, myxol-2'-dimethylfucoside (myxoxanthophyll), zeaxanthin, 39-hydroxy-echinenone, cis-zeaxanthin, echinenone and b-carotene (Graham and Bryant, 2008).…”