A mechanism for red coloration in vertebrates

Red coloration is a salient feature of the natural world. Many vertebrates
produce red color by converting dietary yellow carotenoids into red
ketocarotenoids via an unknown mechanism. Here, we show that two enzymes,
cytochrome P450 2J19 (CYP2J19) and 3-hydroxybutyrate dehydrogenase 1-like
(BDH1L), are sufficient to catalyze this conversion. In birds, both
enzymes are expressed at sites of ketocarotenoid biosynthesis (feather
follicles and red cone photoreceptors), and genetic evidence implicates
these enzymes in yellow/red color variation in feathers. In fish, homologs
of CYP2J19 and BDH1L are required for ketocarotenoid production, and we
show that these enzymes are sufficient to produce ketocarotenoids in cell
culture and when ectopically expressed in fish skin. Finally, we
demonstrate that the red-cone-enriched tetratricopeptide repeat protein
39B (TTC39B) enhances ketocarotenoid production when co-expressed with
CYP2J19 and BDH1L. The discovery of this mechanism of ketocarotenoid
biosynthesis has major implications for understanding the evolution of
color diversity in vertebrates.