Carotenoids in Cyanobacteria

Carotenoids in cyanobacteria have two main functions: they serve as light-harvesting pigments in photosynthesis, and they protect against photooxidative damage. Carotenoids are generally hydrophobic isoprenoid compounds that are synthesized in membranes. They mostly accumulate in protein complexes in the photosynthetic membrane, in the cell membrane and in the cell wall. In addition to the prevailing β-carotene and zeaxanthin, cyanobacteria contain unique ketocarotenoids such as echinenone and canthaxanthin. They do not synthesize β-rings and therefore contain β-, but not α- or δ-, carotenes and their oxygenated forms. Glycosylated carotenoids are also very common in cyanobacteria. In the biosynthesis pathway of carotenoids, four enzymes convert geranylgeranyl pyrophosphate of the central isoprenoid pathway to β-carotene. It has been established that phytoene synthase carries out the two-step conversion of geranylgeranyl pyrophosphate to phytoene; phytoene desaturase catalyzes the dehydrogenation of phytoene and phytofluene to produce ζ-carotene; ζ-carotene desaturase produces lycopene via neurosporene; and lycopene cyclase catalyzes two cyclization reactions that produce γ- and β-carotene. Genes encoding several carotenogenic enzymes have been cloned from cyanobacteria. Their analysis has revealed some of the molecular mechanisms involved in carotenoid biosynthesis. The primary structures of the polypeptides are conserved with the homologous enzymes in algae and plants but are distinct from those of other microorganisms. Each of these enzymes is a single-gene produce that is functional in an autonomous manner in heterologous cells. The carotenoid biosynthesis pathway is a target to various ‘bleaching herbicides.’ Resistance to inhibitors of phytoene desaturase was found to be induced by mutations that lead to amino acid substitutions in the enzyme phytoene desaturase. Synthesis and accumulation of carotenoids is regulated by growth conditions and environmental signals. Very little is known about the molecular mechanisms that govern these processes.