The enzyme, characterized from the cyanobacteria?Nostoc?sp. PCC 7120 and?Mastigocladus laminosus, catalyses the covalent attachment of the phycobilin chromophore phycocyanobilin to cysteine 84 of the ¦Â subunit of the phycobiliprotein phycoerythrocyanin and its isomerization to phycoviolobilin.