Bixin, or annatto, is one of the oldest natural pigments used by humans. It is currently employed in the manufacture of cosmetics and as a soluble color additive for food, but a consumer ban on azo dye—a chemically synthesized alternative—has increased the demand for bixin. This red pigment is synthesized by Bixa orellana, a plant native to tropical America, and its precursor is thought to be the carotenoid lycopene. In the June 27 Science, Florence Bouvier and colleagues at the Institut de Biologie Moléculaire des Plantes investigate the pathway for bixin synthesis and show that lycopene is subject to remodeling in a sequential reaction by a dioxygenase, an aldehyde dehydrogenase, and a methyltransferase (Science, 300:2089-2091, June 27, 2003).

Bouvier et al. screened a B. orellana cDNA library and isolated a putative lycopene dioxygenase, a bixin aldehyde dehydrogenase, and a methyltransferase, all with a high degree of identity with other plant homologues. Recombinant proteins were capable of catalyzing the sequential conversion of lycopene into bixin. Using Escherichia coli transformed with an expression vector containing all three genes, they achieved production of bixin at 5 mg/g dry weight—providing a system to expand the production of this economically important pigment.

"Given the feasibility of engineering bixin in a heterologous host such as E. coli, we assume that coexpressing the three cloned genes in sink organs such as tomato fruit, which accumulate massive amounts of the necessary precursor lycopene, should lead to an alternative and competitive source for natural bixin production," conclude the authors.