Identification of a carotenoid oxygenase synthesizing acyclic xanthophylls: combinatorial biosynthesis and directed evolution.

A carotenoid desaturase homolog from Staphylococcus aureus (CrtOx) was identified. When expressed in engineered E. coli cells synthesizing linear C(30) carotenoids, polar carotenoid products were generated, identified as aldehyde and carboxylic acid C(30) carotenoid derivatives. The major product in this engineered pathway is the fully desaturated C(30) dialdehyde carotenoid 4,4'-diapolycopen-4,4'-dial. Very low carotenoid yields were observed when CrtOx was complemented with the C(40) carotenoid lycopene pathway. But extension of an in vitro evolved pathway of the fully desaturated 2,4,2',4'-tetradehydrolycopene produced the structurally novel fully desaturated C(40) dialdehyde carotenoid 2,4,2',4'-tetradehydrolycopendial. Directed evolution of CrtOx by error-prone PCR resulted in a number of variants with higher activity on C(40) carotenoid substrates and improved product profiles. These findings may provide new biosynthetic routes to highly polar carotenoids with unique spectral properties desirable for a number of industrial and pharmaceutical applications.