Functional analysis of genes from Streptomyces griseus involved in the synthesis of isorenieratene, a carotenoid with aromatic end groups, revealed anovel type of carotenoid desaturase

The biosynthesis of the aromatic carotene isorenieratene is restricted to g reen photosynthetic bacteria and a few actinomycetes. Among them Streptomyc es griseus has been used to study the genes involved in this pathway. Five genes out of seven of two adjacent operons in one cluster could be identifi ed to be sufficient for the synthesis of isorenieratene. Stepwise deletions of these genes demonstrated their participation in phytoene synthesis, phy toene desaturation and lycopene cyclization. The novel gene crtU was assign ed to encode a unique desaturase responsible for the conversion of beta-car otene via beta-isorenieratene to isorenieratene by a desaturation/methyltra nsferation mechanism. Sequence analysis of crtU revealed two conserved regi ons, one at the N-terminus and the other at the C-terminus of the protein w hich is universal to different types of carotene desaturases. In addition, the sequence comprises a motif typically found in methyltransferases. The d eletion of the two remaining genes of the cluster left the carotenoid biosy nthetic pathway unaffected. (C) 1999 Elsevier Science B.V. All rights reser ved.