Metabolic engineering for the production of carotenoids in non-carotenogenic bacteria and yeasts.

The crt gene clusters responsible for the biosynthesis of carotenoids such as lycopene, beta-carotene and astaxanthin have been isolated from carotenogenic bacteria such as Erwinia species and the marine bacterium Agrobacterium aurantiacum. The functions of the individual genes have been identified. The first substrate of the enzymes encoded by the Erwinia crt clusters is farnesyl pyrophosphate which is not only the precursor for carotenoid biosynthesis but also sterols, dolichols and other numerous isoprenoid compounds. Escherichia coli does not naturally synthesize carotenoids, but by using the carotenogenic genes recombinant strains accumulating lycopene, beta-carotene and astaxanthin have been produced. Other non-carotenogenic bacteria such as Zymomonas mobilis have also been engineered to produce beta-carotene by the introduction of the corresponding crt genes. A gene capable of enhancing carotenoid levels in E. coli has also been isolated from cDNA libraries of the yeast Phaffia rhodozyma and the green alga Haematococcus pluvialis. This gene has been found to encode an isopentenyl pyrophophate isomerase. It has further been shown that the edible yeasts Candida utilis as well as Saccharomyces cerevisiae, which possess no carotenoid biosynthetic pathway, acquire the ability to produce carotenoids, when the carotenogenic genes are expressed under the control of yeast-derived promoters and terminators. It has been observed in the yeasts S. cerevisiae and C. utilis carrying the lycopene biosynthesis genes that ergosterol content is decreased by 10 and 35%, respectively. It is therefore likely that the carbon flux for the ergosterol biosynthesis has been partially directed from farnesyl pyrophosphate to a new pathway for the lycopene biosynthesis. Further, the expression of a truncated gene which codes for the catalytic domain of the endogenous 3-hydroxy-3-methylglutaryl coenzyme. A reductase, has been found to be effective for enhancing carotenoid levels in the yeast C. utilis.