Isorenieratene Biosynthesis in Green Sulfur Bacteria Requires the Cooperative Actions of Two Carotenoid Cyclases

Abstract

The cyclization of lycopene to ␥-or ␤-carotene is a major branch point in the biosynthesis of carotenoids in photosynthetic bacteria. Four families of carotenoid cyclases are known, and each family includes both monoand dicyclases, which catalyze the formation of ␥-and ␤-carotene, respectively. Green sulfur bacteria (GSB) synthesize aromatic carotenoids, of which the most commonly occurring types are the monocyclic chlorobactene and the dicyclic isorenieratene. Recently, the cruA gene, encoding a conserved hypothetical protein found in the genomes of all GSB and some cyanobacteria, was identified as a lycopene cyclase. Further genomic analyses have found that all available fully sequenced genomes of GSB encode an ortholog of cruA. Additionally, the genomes of all isorenieratene-producing species of GSB encode a cruA paralog, now named cruB. The cruA gene from the chlorobactene-producing GSB species Chlorobaculum tepidum and both cruA and cruB from the brown-colored, isorenieratene-producing GSB species Chlorobium phaeobacteroides strain DSM 266 T were heterologously expressed in lycopene-and neurosporene-producing strains of Escherichia coli, and the cruB gene of Chlorobium clathratiforme strain DSM 5477 T was also heterologously expressed in C. tepidum by inserting the gene at the bchU locus. The results show that CruA is probably a lycopene monocyclase in all GSB and that CruB is a ␥-carotene cyclase in isorenieratene-producing species. Consequently, the branch point for the synthesis of mono-and dicyclic carotenoids in GSB seems to be the modification of ␥-carotene, rather than the cyclization of lycopene as occurs in cyanobacteria.

Isolation and Characterization of Homodimeric Type-I Reaction Center Complex from Candidatus Chloracidobacterium thermophilum, an Aerobic Chlorophototroph

“…The synthesis of deoxyflexixanthin from lycopene requires a lycopene monocyclase (either CruA or CrtYcYd), CrtC, CrtD, and a 4-ketolase (CrtO). The presence of genes for two lycopene cyclases suggests that one may act preferentially as a monocyclase, whereas the other enzyme is either a bicyclase or preferentially adds a second ring to ␥-carotene like CruB in BChl e-containing GSB strains (56). The complement of genes for carotenogenesis in Ca.…”

Sulfur globule oxidation in green sulfur bacteria is dependent on the dissimilatory sulfite reductase system

“…Plasmids for genetic complementation of the dsrM mutant of C. tepidum were constructed using the bchU gene as expression platform, similar to the manner described by Maresca et al (2008). Two homologous regions (A and B) flanking the start codon of the C. tepidum bchU gene were amplified by PCR using primers 486 and 487 for region A (865 bp), and primers 488 and 489 for region B (864 bp) (Supplementary Table S2).…”

Identification of the Bacteriochlorophylls, Carotenoids, Quinones, Lipids, and Hopanoids of "Candidatus Chloracidobacterium thermophilum"

“…Chlorobactene, an aromatic derivative of ␥-carotene, is the major carotenoid found in chlorosomes of C. tepidum (15,17). However, browncolored, BChl e-containing GSB such as Chlorobium phaeobacteroides usually produce aromatic bicyclic carotenoids, isorenieratene and ␤-isorenieratene, instead of chlorobactene (32,48,49,64). In addition to chlorobactene or isorenieratene, both greenand brown-colored GSB also synthesize small amounts of glycosylated carotenoids (47,65).…”