We have isolated, based on the knowledge of the complete genomic sequence of the cyanobacterium Synechocystis sp. PCC 6803, an open reading frame (slr0088) similar to known bacterial carotene desaturases and have analyzed the function of the encoded protein. Surprisingly, this protein has no detectable desaturase activity with phytoene, hydroxyneurosporene, or -carotene as substrates, but is rather a ␤-carotene ketolase that acts asymmetrically introducing a keto group on only one of the two ␤-ionone rings of ␤-carotene to generate echinenone. This is in contrast to the so far characterized ␤-carotene ketolases that act symmetrically, producing the di-keto carotenoid canthaxanthin from ␤-carotene without significant accumulation of echinenone. We have designated this new gene crtO. The function of the crtO gene product has been demonstrated by 1) the biosynthesis of echinenone when the crtO gene is expressed in an Escherichia coli strain able to accumulate ␤-carotene, 2) the in vitro biosynthesis of echinenone from ␤-carotene with cell free extracts from E. coli cells that express the crtO gene, and 3) the absence of echinenone in a Synechocystis strain in which the crtO gene has been insertionally inactivated. The primary structure of the Synechocystis asymmetric ketolase bears no similarity with the known ␤-carotene ketolases. crtO is not required for normal growth under standard or high light conditions, neither is the photosynthetic activity of the crtO-deficient strain affected.

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