The sequence of a nucleic acid isolated from a cDNA library of the flowering plant Adonis aestivalis is disclosed (SEQ ID NO: 1). This DNA sequence, referred to as AdKC28, encodes for a protein that acts in conjunction with proteins encoded by either one of two other closely-related Adonis aestivalis cDNAs, AdKeto1 and AdKeto2, to convert .beta.-carotene (.beta.,.beta.-carotene) into astaxanthin (3,3'-dihydroxy-4,4'-diketo-.beta.,.beta.-carotene). Together, these Adonis aestivalis cDNAs, when operably linked to promoters appropriate to the transgenic host, enable the production of astaxanthin and other carotenoids with 3-hydroxy-4-keto-.beta.-rings in a variety of host cells and organisms.