A carotenoid biosynthesis gene cluster for the production of astaxanthin was isolated
from the marine bacterium Agrobacterium aurantiacum. This cluster contained five carotenogenic
genes with the same orientation, which were designated crtW, crtZ, crtY, crtI, and
crtB. The stop codons of individual crt genes except for crtB overlapped the start
codons of the following crt genes. Escherichia coli transformants carrying the Erwinia
uredovora carotenoid biosynthesis genes provide suitable substrates for carotenoid
biosynthesis. The functions of the five crt genes of A. aurantiacum were determined
through chromatographic and spectroscopic analyses of the pigments accumulated in
some E. coli transformants carrying various combinations of the E. uredovora and A.
aurantiacum carotenogenic genes. As a result, the astaxanthin biosynthetic pathway
is proposed for the first time at the level of the biosynthesis genes. The crtW and
crtZ gene products, which mediated the oxygenation reactions from beta-carotene to
astaxanthin, were found to have low substrate specificity. This allowed the production
of many presumed intermediates of astaxanthin, i.e., adonixanthin, phoenicoxanthin
(adonirubin), canthaxanthin, 3'-hydroxyechinenone, and 3-hydroxyechinenone.