The thermophilic cyanobacterium, Synechococcus elongatus, has emerged as a model organism for structural
studies of photosynthetic complexes. It has provided 3D crystals for the first determinations of the
structures of photosystem I (PSI), initially at 4 Angstroms, but now at 2.5 Angstroms (Krauss et al., 1996; Jordan et al., 2001), and of photosystem II (PSII) at 3.8 Angstroms resolution by X-ray diffraction (Zouni et al., 2001). As is the case with other cyanobacteria, S. elongatus also contains extrinsic, soluble, macromolecular structures known as phycobilisomes, which act as a light-harvesting system for PSII and to some extent for PSI (Rogner et al., 1996).

The S. elongatus phycobilisome consists of the phycobiliproteins, allophycocyanin (A-PC) and C-phycocyanin
(C-PC), together with linker proteins (Sidler, 1994). The bilin pigments are open-chained tetrapyrroles, covalently linked to apoproteins. We have isolated C-PC from S. elongatus and by X-ray diffraction analysis of
3D crystals obtained its structure at 1.45 Angstrom. This has a better resolution than any other previous structure of CPC and indeed of any phycobiliprotein determined to date, achieved through the application of a new crystallization method. Recent reports describe the crystallization (Toriumi et al., 2001) of S. elongatus C-PC and the structures of Synechococcus vulcanus C-PC at 2.5 Angstrom resolution (Adir et al., 2001) and Spirulina platensis CPC at 2.2 Angstrom resolution (Padyana et al., 2001; Wang et al., 2001). Our comparison with the previous most highly resolved C-PC structures from the thermophile Cyanidium caldarium at 1.65 Angstrom resolution (Stec et al., 1999 and the nonthermophile Fremyella diplosiphon at 1.66 Angstrom resolution (Duerring et al., 1991) reveals differences in chromophore conformation. The significance of this is discussed in terms of intra- and interchromophore distances and energy transfer pathways.