Biosynthesis of ketocarotenoids in transgenic cyanobacteria expressing the algal gene for beta-C-4-oxygenase, crtO.

The ketocarotenoid astaxanthin is produced by a number of marine bacteria and microalgae. It is synthesized from beta-carotene by the addition of two keto groups to carbons C4 and C4' and two hydroxyl groups to C3 and C3'. The gene, crtO, encoding beta-C-4-oxygenase which converts beta-carotene to canthaxanthin was cloned from the green alga Haematococcus plurialis. We transferred crtO to the cyanobacterium Synechococcus PCC7942, which contains a beta-carotene hydroxylase gene and normally accumulates beta-carotene and zeaxanthin. The genetically engineered cyanobacterium produced astaxanthin as well as other ketocarotenoids. The results confirm that crtO can function in cyanobacteria in conjunction with the intrinsic carotenoid enzymes to produce astaxanthin. Specifically, this finding indicates that beta-carotene hydroxylase, which normally converts beta-carotene to zeaxanthin, can also function in the biosynthesis of astaxanthin. These results provide the first evidence of genetic manipulation of a plant-type carotenoid biosynthesis pathway toward the production of novel carotenoids.