Novel β-carotene ketolases from non-photosynthetic bacteria for canthaxanthin synthesis

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Abstract
We reported previously that the Rhodococcus erythropolis strain AN12 synthesizes the monocyclic carotenoids 4-keto γ-carotene and γ-carotene. We also identified a novel lycopene β-monocyclase in this strain. Here we report the identification of the rest of the carotenoid synthesis genes in AN12. Two of these showed apparent homology to putative phytoene dehydrogenases. Analysis of Rhodococcus knockout mutants suggested that one of them ( crtI) encodes a phytoene dehydrogenase, whereas the other ( crtO) encodes a β-carotene ketolase. Expression of the β-carotene ketolase gene in an Escherichia coli strain which accumulates β-carotene resulted in the production of canthaxanthin. In vitro assays using a crude extract of the E. coli strain expressing the crtO gene confirmed its ketolase activity. A crtO homologue (DR0093) from Deinococcus radiodurans R1 was also shown to encode a β-carotene ketolase, despite its sequence homology to phytoene dehydrogenases. The Rhodococcus and Deinococcus CrtO ketolases both catalyze the symmetric addition of two keto groups to β-carotene to produce canthaxanthin. Even though this activity is similar to the CrtW-type of ketolase activity, the CrtO ketolases show no significant sequence homology to CrtW-type ketolases. The presence of six conserved regions may be a signature for the CrtO-type of β-carotene ketolases.