[5][a] The CRT gene cluster consists of twenty-five genes such as crtA, crtB, crtC, crtD, crtE, crtF, crtG, crtH, crtI, crtO, crtP, crtR, crtT, crtU, crtV, and crtY, crtZ.
crtY cyclizes lycopene into β-carotene, which is subsequently oxygenated by crtW to form canthaxanthin.
[1] Hydroxylation of Zeaxanthin occurs by β-carotene hydroxylase an enzyme encoded on the crtR (in cyanobacteria) and crtZ gene (in Chlorophyta).
[11][12]CrtZ, similarly to crtO, is also capable of converting carotenoids into β-cryptoxanthin, Zeaxanthin, 3-Hydroxyechinenone, 3'-Hydroxyechinenone, Astaxanthin, Adonixanthin, and Adonirubin.
[2] crtG encodes for carotenoid 2,2'- β-hydroxylase, this enzyme leads to the formation of 2-hydroxylated and 2,2′-dihydroxylated products in E coli.
[1] The less conserved nature of these genes allowed for the expansion of the carotenoid biosynthesis pathway and its end products.
Amino acid variations within crt genes have evolved due to purifying and adaptive selection.