Production and functional evaluation of cyclic carotenoids in the purple bacterium, Rubrivivax gelationsus

Cyclic carotenoids were synthesized in transformed strains of the purple phototrophic bacterium Rubrivivax gelatinosus through heterologous expression of the lycopene cyclase gene ( crtY ), the β-carotene ketolase gene ( crtW ), and/or the β-carotene hydroxylase ( crtZ ) gene. Significant proportions of the synthesized cyclic carotenoids were found to associate with the light-harvesting complexes of the transformed strains. Using these engineered strains, the in vivo protective roles of cyclic carotenoids against photodamage and singlet oxygen were evaluated and compared with those of acyclic carotenoids that are characteristic of purple phototrophic bacteria. Strains containing cyclic carotenoids generally exhibited higher viability in the presence of singlet oxygen than strains producing only acyclic carotenoids. In contrast, cyclic carotenoids—except for zeaxanthin—displayed lower efficiency in quenching the excited states of bacteriochlorophylls compared with acyclic carotenoids.