Figure 5

Proposed bicyclic and monocyclic carotenoid biosynthesis pathways in P. rhodozyma. The starting compound, acetyl-CoA, passes through the mevalonate pathway for synthesis of isopentenyl pyrophosphate (IPP) and farnesyl pyrophosphate (FPP) as isoprenoid precursors. The following bicyclic pathway involves sequential synthesis of, GGPP, phytoene, lycopene, and β-carotene by a series of catalytic enzymes encoded respectively by crtE, crtYB, and crtI. The recently proposed monocyclic pathway diverges from the bicyclic pathway at neurosporene, with HDCO as the primary product, produced through β-zeacarotene, γ-carotene, and torulene. Results of the present study suggested that CrtS functions in both the bicyclic and monocyclic pathways. Increased levels or activity of CrtS allow HDCO to be used as a substrate for further enhancement of astaxanthin yield (dashed line at bottom).