The gamma subunits of phycoerythrin from a red alga: Position in phycobilisomes and sequence characterization

Aglaothamnion neglectum Feldman-Mazoyer has two gamma subunits, gamma (31) and gamma (33), that are associated with phycoerythrin in the light-harvest ing phycobilisomes. We demonstrate that these subunits are spatially separa ted within the phycobilisome, with the gamma (31) subunit present at the di stal end of phycobilisome rods and the gamma (33) subunit present on the pr oximal end. These subunits are thought to link phycoerythrin hexamers toget her in the rod substructure, serving a role analogous to that of linker pol ypeptides of cyanobacteria (although unlike the cyanobacterial Linker polyp eptides they are chromophorylated), The sequencing of tryptic polypeptides of the gamma subunits enabled us to prepare oligonucleotides encoding diffe rent regions of gamma (31) These oligonucleotides were used as primers to g enerate a probe for isolating a gamma (31) cDNA clone. Characterization of the cDNA clone predicts a polypeptide of 280 amino acids with a 42 amino ac id presequence that is characteristic of a transit peptide, the peptide tha t targets proteins to chloroplasts of vascular plants, The gamma (31) subun it has 50% similarity to the previously characterized gamma (33) subunit bu t has no identifiable similarity to functionally related polypeptides prese nt in cyanobacterial phycobilisomes or to any other polypeptides in the dat abases. A repeat of 95 amino acids is present in the red algal gamma subuni t sequences, suggesting that these proteins were generated by a gene duplic ation followed by fusion of the duplicate sequences.