The genes encoding the alpha and beta subunits of allophycocyanin, phycocyanin 
and phycoerythrin from the red alga Aglaothamnion neglectum were isolated and 
characterized. While the operons containing the different phycobiliprotein genes 
are dispersed on the plastid genome, the genes encoding the alpha and beta 
subunits for each phycobiliprotein are contiguous. The beta subunit gene is 5' 
for both the phycocyanin and phycoerythrin operons, while the alpha subunit gene 
is 5' for the allophycocyanin operon. The amino acid sequences of A. neglectum 
phycobiliproteins, as deduced from the nucleotide sequences of the genes, are 
65-85% identical to analogous proteins from other red algae and cyanobacteria. 
The conserved nature of the plastid-encoded red algal and cyanobacterial 
phycobiliprotein genes supports the proposed origin of red algal plastids from 
cyanobacterial endosymbionts. Many environmental factors effect phycobilisome 
biosynthesis. The effect of both nutrient availability and light quantity on the 
level of A. neglectum phycobiliprotein subunits and the mRNA species encoding 
those subunits is described.