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R-phycoerythrin, a light-harvesting component from the red algae Gracilaria chilensis, was crystallized by vapour diffusion using ammonium sulfate as precipitant agent. Red crystals grew after one week at 293 K and diffracted to 2.70 Å resolution. Three serial macroseeding assays were necessary to grow a second larger crystal to dimensions of 0.68 × 0.16 × 0.16 mm. This crystal diffracted to 2.24 Å resolution using synchrotron radiation at beamline BM14 of the European Synchrotron Radiation Facility (ESRF) at Grenoble, France and was used for structure determination. Data were collected at 100 K to a completeness of 98.6%. The crystal was trigonal, space group R3, with unit-cell parameters a = b = 187.3, c = 59.1 Å, α = β = 90, γ = 120°. Data treatment using the CCP4 suite of programs indicated that the crystal was twinned (〈I2〉/〈I〉2 = 1.41). Molecular replacement was performed with AMoRe using the R-phycoerythrin from Polysiphonia urceolata [Chang et al. (1996), J. Mol. Biol. 249, 424–440] as a search model. In order to overcome the twinning problem, SHELX97 was used for the crystallographic refinement. The twin fraction was 0.48, indicating a nearly perfect hemihedrally twinned crystal. The final Rwork and Rfree factors are 0.16 and 0.25, respectively. All the residues and chromophores of the α- and β-chains are well defined in the electron-density maps. Some residues belonging to the γ-linker are also recognizable.
