CLONING AND FUNCTIONAL EXPRESSION IN ESCHERICHIA-COLI OF A CYANOBACTERIAL GENE FOR LYCOPENE CYCLASE, THE ENZYME THAT CATALYZES THE BIOSYNTHESIS OF BETA-CAROTENE
Fx. Cunningham et al., CLONING AND FUNCTIONAL EXPRESSION IN ESCHERICHIA-COLI OF A CYANOBACTERIAL GENE FOR LYCOPENE CYCLASE, THE ENZYME THAT CATALYZES THE BIOSYNTHESIS OF BETA-CAROTENE, FEBS letters, 328(1-2), 1993, pp. 130-138
Carotenoids with cyclic end groups are essential components of the pho
tosynthetic membrane in all known oxygenic photosynthetic organisms. T
hese yellow pigments serve the vital role of protecting against potent
ially lethal photo-oxidative damage. Many of the enzymes and genes of
the carotenoid biosynthetic pathway in cyanobacteria, algae and plants
remain to be isolated or identified. We have cloned a cyanobacterial
gene encoding lycopene cyclase, an enzyme that converts the acyclic ca
rotenoid lycopene to the bicyclic molecule beta-carotene. The gene was
identified through the use of an experimental herbicide, 244-methylph
enoxy)triethylamine hydrochloride (MPTA), that prevents the cyclizatio
n of lycopene in plants and cyanobacteria. Chemically-induced mutants
of the cyanobacterium Synechococcus sp. PCC7942 were selected for resi
stance to MPTA, and a mutation responsible for this resistance was map
ped to a genomic DNA region of 200 bp by genetic complementation of th
e resistance in wild-type cells. A 1.5 kb genomic DNA fragment contain
ing this MPTA-resistance mutation was expressed in a lycopene-accumula
ting strain of Escherichia coli. The conversion of lycopene to beta-ca
rotene in these cells demonstrated that this fragment encodes the enzy
me lycopene cyclase. The results indicate that a single gene product,
designated lcy, catalyzes both of the cyclization reactions that are r
equired to produce beta-carotene from lycopene, and prove that this en
zyme is a target site of the herbicide MPTA. The cloned cyanobacterial
lcy gene hybridized well with genomic DNA from eukaryotic algae, thus
it will enable the identification and cloning of homologous genes for
lycopene cyclase in algae and plants.