Antibodies raised against mixtures of phycobilisome polypeptides from the 
eukaryotic alga Cyanidium caldarium were used in an immunological screen to 
detect expression of phycobiliprotein genes in an Escherichia coli library 
containing segments of plastid (chloroplast, cyanelle) DNA from another 
eukaryotic alga, Cyanophora paradoxa. The four candidate clones obtained were 
mapped by restriction analysis and found to be overlapping. The clone with the 
smallest insert (1.4 kilobases) was partially sequenced and a coding region 
similar to the carboxyl terminus of the phycobiliprotein subunit 
beta-phycocyanin was found. The coding region for the beta-phycocyanin gene in 
C. paradoxa has been mapped to the small single copy region on the cyanelle 
genome, and its orientation has been determined. A short probe unique to a 
conserved chromophore binding site shared by at least two phycobiliprotein 
subunits has now been generated from the carboxyl terminus of the 
beta-phycocyanin gene. This probe may be useful in identifying specific 
phycobiliprotein subunit genes, beta-phycocyanin, beta-phycoerythrocyanin, and 
possibly beta-phycoerythrin, in other eukaryotic algae and in prokaryotic 
cyanobacteria.