The gamma subunit of R-phycoerythrin and its possible mode of transport into the plastid of red algae

Abstract

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.