We examined the viability of Escherichia coli transformants harboring various carotenoids synthesizing genes in a medium containing an enzymatic singlet oxygen generating system, which contained myeloperoxidase, hydrogen peroxide and Br- at pH 4,5, Singlet oxygen quenching activities of various carotenoids in phosphatidyl choline micelles in aqueous media were also studied using the same enzymatic singlet oxygen generating system. Viability of the transformants producing carotenoids mas higher than that of the wild type E. coli in the singlet oxygen generation mixture. Of the transformants tested, the viability of zeaxanthin-diglucoside producing transformant was the highest. Carotenoids in increasing order of k(q) values were beta -carotene, a cyclic carotene < zeaxanthin with hydroxy groups <less than or equal to> lycopene, an acyclic carotene (canthaxanthin and astaxanthin with keto groups much less than zeaxanthin-diglucoside. The k(q) value of zeaxanthin-diglucoside was 3.5 times higher than that of beta -carotene. These results suggest that orientation of the carotenoids in lipid layers of micelles and also in phospholipid membrane of bacteria is important for quenching of singlet oxygen, Furthermore, the viability of transformants producing lycopene and phytoene mas almost as high as that of the transformant producing zeaxanthin-glucoside. (C) 2000 Federation of European Biochemical Societies. Published by Elsevier Science E,V, All rights reserved.