2. Materials and methods
2.1. C-phycocyanin isolation and purification
Synechococcus elongatus cultures were grown at 57C in a 30 liter air-lift fermenter and cells broken as detailed previously (Nield et al., 2000). Unbroken cells were precipitated by a 2-min, 3000g centrifugation step in 10mM Mes, pH 7.0, after which the supernatant was further centrifuged at 37,500g for 15 min. The subsequent free phycocyanin-containing phycobilisome material remaining in the supernatant was concentrated to ~70mgml-1 protein using Amicon Centricon Y-100 microconcentrators at 1500g. Gradient SDS–PAGE (10–17%) containing 6M urea was performed to verify the protein composition and the protein bands were visualized using Coomassie R-250 staining.

2.2. Crystallization
Crystals were grown at 20C in hanging drops, using Linbro plates containing 1ml of reservoir solutions. One
microliter of protein solution at 20mgml-1 was mixed with 1 ul of reservoir solution to form the protein drops.
Large single crystals were obtained by separation of the nucleation and growth phases of crystallization. This
was achieved by incubating the drops for 7 h over reservoirs at nucleation conditions consisting of 1.1M ammonium sulfate and 40mM Mes, pH 6.1 and then ‘‘backing-off’’ into metastable conditions by transferring
the drops over reservoirs at 0.6M ammonium sulfate (same buffer). The crystals were obtained within 1 week
of transferring the coverslips.

2.3. Crystallographic data collection
Crystals were transferred from the hanging drops into a cryo-protectant solution containing 25% ethylene
glycol, 0.7M AS, and 0.1M Mes, pH 6.1. The crystals were transferred rapidly (total residence time typically
1 min) into a cryostream of dry nitrogen gas at 100 K. Data were collected with the rotation method at Station
14.1 of the Synchrotron Radiation Source (SRS) (Daresbury, UK), using an ADSC Quantum 4 CCD detector system. The wavelength (k) was set to 1.244 A. The total number of frames recorded was 60, each covering 1 degree of rotation. Reflection intensities were estimated with the MOSFLM package (Leslie, 1987), and
the data scaled, reduced, and analyzed with the CCP4 package (Collaborative Computational Project, 1994).

2.4. Structure solution and refinement
The refined model of C-PC from F. diplosiphon (Krauss et al., 1996) was used as a search probe in AMoRe (Navaza, 1994). The final fitted coordinates from AMoRe were checked for close contacts using the graphics program "O" (Jones and Kieldgaard, 1993). The MR solution was SIGMAA-weighted to account for absent di.racting material (Read, 1986). The model sequence was adjusted with "O" to match the published sequence, prior to re.nement with REFMAC (Murshudov et al., 1997). Manual adjustment of the model was performed at various stages. The chromophore model was also adjusted to match the electron-density map. Remaining positive density was filled, as appropriate, with solvent molecules. The thermal parameters were allowed to refine, but were restrained to be similar to those of neighboring atoms (Table 1). Toward the latter stages of the re.nement atoms were allowed to refine anisotropically. The H atoms were included in their calculated positions, but were not re.ned independently and were not kept in the output set of coordinates. Seventeen side-chains were identi.ed as having more than one conformation and were modeled with 2 conformations each having an occupancy of 0.5. The validity of the occupancy assignment was checked only by thermal parameters having very similar values.