CrtI

CrtI, short for carotenoid desaturase I, is an enzyme widely found in bacteria and some archaea that catalyzes the desaturation of phytoene, the first colorless carotenoid, as part of the carotenoid biosynthesis pathway. The crtI gene encodes this enzyme and is commonly located in carotenoid gene clusters along with crtB (phytoene synthase) and crtY (lycopene cyclase).

CrtI is responsible for converting phytoene to lycopene by introducing multiple double bonds; depending on the organism, the enzyme can perform all steps in one continuous reaction or proceed via intermediates such as zeta-carotene and neurosporene. The final product, lycopene, is a red pigment and serves as a crucial precursor for a variety of other carotenoids.

In many bacteria CrtI is membrane-associated and relies on cellular electron transfer components to drive desaturation. The CrtI family is diverse, and different crtI genes can yield variation in desaturation steps and intermediate profiles, influencing the carotenoid composition of the host organism. Because of its central role, crtI is a common target in metabolic engineering and synthetic biology; heterologous expression of crtI in engineered microbes enables microbial production of lycopene for nutritional supplements and industrial carotenoid synthesis.

CrtI is part of a broader set of carotenoid biosynthetic genes. In strains lacking native CrtI, alternative desaturases or desaturation pathways may exist, but CrtI remains a well-studied enzyme for introducing conjugated double bonds characteristic of colorful carotenoids.