The orange-head Chinese cabbage attract the interest of the consumers and breeders because it accumulate higher amounts of carotenoids and lycopene in comparison with the normal white head cabbage.The orange head trait is controlled by a single recessive gene,Br-or.High resolution genetic maps were successively constructed by Liu,Wang,Zhang,and Feng and some SCAR,RAPD and SSR markers which linked to Br-or distantly were developed,these works make an important contribution to marker-assisted selection breeding.However the markers still have certain genetic distance to the target gene and the molecular mechanism of carotenoids accumulation in orange head cabbage was not reported.In this article,we successfully isolated the gene representing Br-or by map-based cloning and verified its function.We showed that Br-or encodes a carotenoid isomerase,Bra031539,and the orange head trait is likely to be a loss-of-function mutation which is mainly caused by DNA fragment insertion at the 3terminal of Bra031539.To study the isomerase activity of Bra031539,we developed an enzymatic in vitro assay in which the purified E.coli-expressed normal-CRTISO and mutational-CRTISO were mixed with crude plant extraction.The result showed that the normal-CRTISO,not the mutational one,can catalyse the formation of all-trans-lycopene.We also learned the expression pattern by the promotor::GUS staining strategy and found Bra031539s expression localization was consisted with carotenoid synthesis position perfectly.Finally,we suggested a considerable role of the feedback-effect of prolycopene,the substrate of BrCRTISO(Bra031539),in regulation of carotenoids biosynthesis.