The structure of R-phycoerythrin (R-PE) from the red alga Griffithsia monilis was solved at 1.90-Angstrom resolution by molecular replacement, using the atomic coordinates of cyanobacterial phycocyanin from Fremyella diplosiphon as a model. The crystallographic R factor for the final model is 17.5% (R-free 22.7%) for reflections in the range 100-1.90 Angstrom. The model consists of an (alpha beta)(2) dimer with an internal noncrystallographic dyad and a fragment of the gamma-polypeptide. The alpha-polypeptide (164 amino acid residues) has two covalently bound phycoerythrobilins at positions alpha 82 and alpha 139. The beta-polypeptide (177 residues) has two phycoerythrobilins bound to residues beta 82 and beta 158 and one phycourobilin covalently attached to rings A and D at;residues beta 50 and beta 61, respectively. The electron density of the gamma-polypeptide is mostly averaged out by threefold crystallographic symmetry, but a dipeptide (Gly-Tyr) and one single Tyr could be modeled. These two tyrosine residues of the gamma-polypeptide are in close proximity to the phycoerythrobilins at position beta 82 of two symmetry-related beta-polypeptides and are related by the same noncrystallographic dyad as the (alpha beta)(2) dimer. Possible energy transfer pathways are discussed briefly. (C) Academic Press.