Peridinin-chlorophyll-protein, a water-soluble light-harvesting complex that has a bluegreen absorbing carotenoid as its main pigment, is present in most photosynthetic dinoflagellates. Its high-resolution (2.0 angstrom) x-ray structure reveals a noncrystallographic trimer in which each polypeptide contains an unusual jellyroll fold of the a-helical amino-and carboxyl-terminal domains. These domains constitute a scaffold with pseudo-twofold symmetry surrounding a hydrophobic cavity filled by two lipid, eight peridinin, and two chlorophyll a molecules. The structural basis for efficient excitonic energy transfer from peridinin to chlorophyll is found in the clustering of peridinins around the chlorophylls at van der Waals distances. Table 1. Crystallographic data. Data collection: Native1 and derivative data (5 IllM K 2 PtCI 4 , 1-day soak) were collected on a rotating anode source (CuKa, 40 kV, 100 mA) with a STOE (Darmstadt, Germany) imaging plate detector. Native2 was collected at the BW7B wiggler bealllline at DESY on a 30-cm MARresearch (Hamburg, Germany) image plate. All data were processed with XDS (26). Phasing: Six heavy-atom binding sites were found using SHELXS (27) in Patterson search mode, and four additional sites were found by inspection of difference Fourier maps. Refinement of heavy-atom parameters was done with DAREFI (28). The phases could be improved further by inclusion of the anomalous signal and the use of solvent flattening and noncrystallographic symmetry averaging of the trimer in the asymmetric unit (29). Model building and refinement: The resulting 2.9 Aelectron density map was readily interpret-able. An initial model of PCP was built with 0 (23). The full sequence could be included, and 10 pigments were modeled. The initial model was refined with X-PLOR (24), including simulated annealing, positional refinement, and manual rebuilding against Native1 and later Native2. Strong noncrystallographic symmetry restraints were applied throughout the refinement. ARP (30) was used to obtain unbiased atomic positions for well-connected density in the 2.0 Adifference electron-density map. The arrangement of these atoms, together with possible hydrogen-bonding patterns, was consistent with an interpretation as DGDG molecules. After refinement, the DGDG head groups obey good stereochemistry and show no difference density. tuolecules. Within each cluster, the efficiency of singlet energy transfer fr01n peridinin to chlorophyll is close to unity (7). Models of chromophore interaction within and atuong the clusters have previously been based on spectroscopic investigations (7,8). Structural infornlation, such as that available for 1uenl-brane-bound LHCs fron1 higher plants (3) and bacteria (9), has greatly enhanced our understanding of antenna systen1s having chlorophyll as the n1ain pignlent. The highresolution structure of PCP gives insight into the highly organized structural basis of lightharvesting by carotenoids and its efficient transfer to chlorophyll and should be of considerable value for re...

The prochlorophytes are oxygenic prokaryotes differing from other cyanobacteria by the presence of a light-harvesting system containing both chlorophylls (Chls) a and b and by the absence of phycobilins. We demonstrate here that the Chl a/b binding proteins from all three known prochlorophyte genera are closely related to IsiA, a cyanobacterial Chl a-binding protein induced by iron starvation, and to CP43, a constitutively expressed Chl a antenna protein of photosystem II. The prochlorophyte Chl a/b protein (pcb) genes do not belong to the extended gene family encoding eukaryotic Chl a/b and Chl a/c light-harvesting proteins. Although higher plants and prochlorophytes share common pigment complements, their light-harvesting systems have evolved independently.