Identification of a fourth family of lycopene cyclases in photosynthetic bacteria

Abstract

A fourth and large family of lycopene cyclases was identified in photosynthetic prokaryotes. The first member of this family, encoded by the cruA gene of the green sulfur bacterium Chlorobium tepidum, was identified in a complementation assay with a lycopene-producing strain of Escherichia coli. Orthologs of cruA are found in all available green sulfur bacterial genomes and in all cyanobacterial genomes that lack genes encoding CrtL-or CrtY-type lycopene cyclases. The cyanobacterium Synechococcus sp. PCC 7002 has two homologs of CruA, denoted CruA and CruP, and both were shown to have lycopene cyclase activity. Although all characterized lycopene cyclases in plants are CrtL-type proteins, genes orthologous to cruP also occur in plant genomes. The CruA-and CruP-type carotenoid cyclases are members of the FixC dehydrogenase superfamily and are distantly related to CrtL-and CrtY-type lycopene cyclases. Identification of these cyclases fills a major gap in the carotenoid biosynthetic pathways of green sulfur bacteria and cyanobacteria.carotenogenesis ͉ carotenoids ͉ cyanobacteria ͉ green sulfur bacteria ͉ photosynthesis C olored carotenoids are found in nearly all photosynthetic species and in a variety of nonphotosynthetic organisms and play roles in light-harvesting, photoprotection, structural maintenance of pigment-protein complexes (1), and membrane structure and fluidity (2). The cyclization of the linear compound lycopene to produce ␣-, ␤-, ␥-, or -carotene is a branch point in carotenoid biosynthetic pathways in many species of bacteria, plants, and fungi (3, 4). The monocyclic ␥-carotene is a precursor to myxoxanthophylls in cyanobacteria (5, 6), is both an intermediate and an end product of carotenogenesis in orange and yellow flowers and fruits (7-10), and is an intermediate in chlorobactene biosynthesis in green sulfur bacteria (GSB) (11-13). Dicyclic ␤-carotene is a major component of photosystems (PS) I and II in cyanobacteria and plants (14,15) and is modified to isorenieratene in brown-colored GSB and some actinomycetes (16,17).Three classes of lycopene cyclases have previously been identified in bacteria: the CrtY-type ␤-cyclases that are found in many carotenogenic proteobacteria (e.g., refs. 18 and 19), Streptomyces spp. (17), and the Chloroflexi; the CrtL family, which includes the ␤-and -cyclases in some cyanobacteria and plants (20,21); and the heterodimeric cyclases of some Gram-positive bacteria (22, 23), which are related to the lycopene cyclases of archaea (24, 25) and halophilic bacteria (26). These classes are distantly related to each other and share only a few conserved motifs, including an N-terminal flavin-binding domain that is found in the first two groups but appears to be missing in the third (27). Carotenoid monocyclases are found in both the CrtY and CrtL families (28-30), and there are no obvious sequence differences between mono-and dicyclases (28).The cyclization of lycopene to ␥-or ␤-carotene is an isomerization reaction, which produces no net change in mass or redox state o...

Lycopene cyclase paralog CruP protects against reactive oxygen species in oxygenic photosynthetic organisms

“…The cyanobacterial Synechococcus sp. PCC 7002 CruP ( Syn CruP) was also shown to convert lycopene into γ-carotene but had lower activity than Ct CruA (8). In general, carotenoid biosynthesis in plants and cyanobacteria is performed by a similar suite of enzymes.…”

“…Recently a fourth family of lycopene cyclases was identified in green sulfur bacteria (GSB) and some cyanobacteria (8). This previously undescribed family of lycopene cyclases is composed of two classes of enzymes, one known as CruA (found in all GSB and cyanobacteria that lack CrtL) and a paralog known as CruP found in cyanobacteria, along with CruA or CrtL, and in higher plants along with CrtL enzymes (LCYB and LCYE).…”

“…In Escherichia coli complementation assays, CruA from the GSB Chlorobium tepidum ( Ct CruA) was shown to convert lycopene into γ-carotene plus small amounts of β-carotene (8). The cyanobacterial Synechococcus sp.…”

“…Lycopene, a linear carotenoid, is the major branch point for the formation of different cyclic carotenoids, such as α-carotene or β-carotene (4). The cyclization of the ends of lycopene is performed by a class of enzymes known as lycopene cyclases (5)(6)(7)(8). In plants, the enzymatic products of the CrtL type lycopene cyclases, lycopene ε-cyclase (LCYE) and lycopene β-cyclase (LCYB), are α-carotene and β-carotene.…”

“…In general, carotenoid biosynthesis in plants and cyanobacteria is performed by a similar suite of enzymes. Although plants already contain two CrtL type lycopene cyclases that form β-and ε-rings, Maresca et al (8) suggested that CruP in plants might be a lycopene cyclase specifically responsible for catalyzing formation of β-rings of α-carotene (8). However, the pigment profile of both the Synechococcus sp.…”