Carotene-superproducing mutants ofPhycomyces blakesleeanus

The wild-type mycelia of the fungusPhycomyces blakesleeanus are yellow because they contain small amounts of β-carotene. Some mutations lead to large increases in β-carotene content. These "deep yellow" mutants carry recessive mutations in either of two genes,carD andcarS, not linked to each other or to other genes related to carotenogenesis. ThecarS mutants contain up to 100 times more β-carotene than the wild type; thecarD mutants, up to 20 times.carS mutants are unable to form zygospores and their carotenogenesis is not activated by retinol; on the other hand,carD mutants complete the sexual cycle and respond to retinol.carS mutations are epistatic overcarD mutations. The product of genecarS mediates the end-product regulation of the pathway; it is suggested that thecarD gene product increases the amount or the activity of thecarS gene product.