Catalyses the four-electron reduction of biliverdin IXα (2-electron reduction at both the A and D rings). Reaction proceeds via an isolatable 2-electron intermediate, 181,182-dihydrobiliverdin. Flavodoxins can be used instead of ferredoxin. The direct conversion of biliverdin IXα (BV) to (3Z)-phycocyanolbilin (PCB) in the cyanobacteria Synechocystis sp. PCC 6803, Anabaena sp. PCC7120 and Nostoc punctiforme is in contrast to the proposed pathways of PCB biosynthesis in the red alga Cyanidium caldarium, which involves (3Z)-phycoerythrobilin (PEB) as an intermediate  and in the green alga Mesotaenium caldariorum, in which PCB is an isolable intermediate.
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