Catalyzes the site-selective attachment of phycocyanobilin (PCB) to 'Cys-82' in CpcB and 'Cys-83' in PecB. Also able to add PCB to ApcA1, ApcA2, ApcB, ApcD and ApcF, but not PecA. In vitro can be made to add phycoerythrobilin, a non-physiological substrate, to its target proteins (using genes from Fremyella diplosiphon PCC 7601). Binds PCB extremely rapidly, followed by a much slower phase of PCB transfer to the apo-acceptor protein. This suggests that the lyase has a chaperone-like function, and may serve to regulate the manner (possibly the conformation) in which the chromophore is attached to its target rather than to accelerate the rate