Accepted name: carotenoid χ-ring synthase
Reaction: carotenoid β-end group + 2 acceptor = carotenoid χ-end group + 2 reduced acceptor
Glossary: okenone = 1′-methoxy-1′,2′-dihydro-χ,ψ-caroten-4′-one
renierapurpurin = χ,χ-carotene
synechoxanthin = χ,χ-caroten-18,18′-dioate
Other name(s): crtU (gene name) (ambiguous); cruE (gene name)
Systematic name: carotenoid β-ring:acceptor oxidoreductase/methyltranferase (χ-ring-forming)
Comments: The enzyme, found in purple sulfur bacteria (Chromatiaceae) and some cyanobacteria, is involved in the biosynthesis of carotenoids that contain χ-type end groups, such as okenone, renierapurpurin, and synechoxanthin.
1.  Graham, J.E. and Bryant, D.A. The Biosynthetic pathway for synechoxanthin, an aromatic carotenoid synthesized by the euryhaline, unicellular cyanobacterium Synechococcus sp. strain PCC 7002. J. Bacteriol. 190 (2008) 7966–7974. [PMID: 18849428]
2.  Vogl, K. and Bryant, D.A. Biosynthesis of the biomarker okenone: χ-ring formation. Geobiology 10 (2012) 205–215. [PMID: 22070388]
[EC created 2018]