R-phycoerythrin is the major light-harvesting pigment protein of most red algal 
phycobilisomes. It is composed of three pigmented polypeptide subunits, the 
alpha, beta, and gamma. While alpha and beta phycoerythrin subunits are each 
unique in the red alga Aglaothamnion neglectum, there are two different gamma 
subunits with distinct molecular masses. Both gamma subunits are pigmented by 
virtue of covalently attached linear tetrapyrroles. The amino acid sequence of 
one of the gamma subunits, as deduced from the nucleotide sequence of a cDNA 
clone, has no significant similarity to any known sequence in the data bases. 
This result is surprising, since the gamma subunit of phycoerythrin is thought 
to have a function that is similar to cyanobacterial linker polypeptides. The A. 
neglectum gamma subunit is synthesized as a 36-kDa precursor protein that is 
processed at the amino terminus to yield a 33-kDa mature protein. The 
amino-terminal extension was able to direct the pea small subunit of Rubisco 
into isolated pea chloroplasts. This result suggests that red algae transport 
proteins into the plastid by a mechanism similar to that of higher plants. There 
are significant changes in levels of mRNA encoding the gamma 33 subunit when A. 
neglectum is grown under different conditions of illumination and in 
nitrogen-deficient medium. These changes parallel those previously observed for 
transcripts encoding the alpha and beta phycoerythrin subunits. Hence, there may 
be coordinated expression of nuclear and plastid-encoded phycoerythrin subunit 
genes.