Abstract
In carotenogenesis, lycopene cyclases catalyze lycopene to β-carotene via γ-carotene. At present, three distinct functional enzymes are known. (1) Bacteria (CrtY), two cyanobacteria (CrtL), and plants (CrtL, CrtL-b) have homologus enzymes. (2) Some bacteria (Mycobacterium, Myxococcus, etc.) have a hetero-dimer of CrtYc and CrtYd, and archaea and fungi have a fused CrtYc and CrtYd monomer. (3) Green sulfur bacterium (Chlorobium tepidum) has CruA, whose homologs have been found from some cyanobacteria.
In this study, functions of cruA-like genes were studied. The disrupted mutant of Anabaena sp. PCC 7120 had no effects on carotenoid composition, while that of Synechocystis sp. PCC 6803 could not be segregated. When the genes from Synechocystis 6803 and Gloeobacter violaceus were expressed in Echerichia coli, which could produce lcopene, β-carotene was not produced. Therefore, cruA-like genes in cyanobacteria have not the functions of lycopene cyclase, and a novel fourth enzyme must be present in cyanobacteria.
