Proc. Natl. Acad. Sci. USA Vol. 76, No. 12, pp. 6162-6166, December 1979 Biochemistry Characterization of cyanobacterial phycobilisomes in zwitterionic detergents (Synechococcus/photosynthetic accessory pigments/sedimentation/electron microscopy/aggregation) ALEXANDER N. GLAZER*, ROBLEY C. WILLIAMSt, GREGORY YAMANAKA*, AND H. K. SCHACHMANt *Department of Microbiology and Immunology, and tDepartment of Molecular Biology, University of California, Berkeley, California 94720 Contributed by Robley C. Williams, September 10, 1979

ABSTRACT Properties of cyanobacterial phycobilisomes preparations of nearly uniform-sized phycobilisomes were (from Synechococcus spp. 6301 and 6312 and Synechocystis sp. obtained. Ultrastructural studies of the particles prepared in 6701) prepared in the presence of two different zwitterionic detergents were compared to those of phycobilisomes detached zwitterionic detergents were facilitated by the marked decrease from membranes with the nonionic detergent Triton X-100 and in aggregation. Such studies show that phycobilisomes from then freed from Triton by sedimentation through high-salt su- different organisms have certain characteristics in common, crose density gradients. The absorption spectra, polypeptide as concluded by others (1, 7, 8), but also exhibit distinctive composition, and ultrastructure of phycobilisomes were inde- structural features. pendent of the detergent used during the preparation. Phyco- bilisomes from certain cyanobacteria aggregated in the absence MATERIALS AND METHODS of detergent. Such aggregation was not seen in preparations containing zwitterionic detergents. Aggregation of phycobili- Strains and Culture Conditions. Synechococcus spp. 6301 somes led to a partial quenching of their fluorescence. Electron (ATCC 27144) and 6312 (ATCC 27167) and Synechocystis sp. microscopy showed the three types of phycobilisomes to have 6701 (ATCC 27170) (9) were grown as 1-liter cultures in me- a hemidisc appearance, although their detailed structures were dium BG-ll (10) with continuous stirring under warm white quite different. fluorescent illumination. Cells were grown at 30°C in the Cells of cyanobacteria and chloroplasts of red algae contain high presence of air or at 40°C gassed with 1% C02/99% N2. concentrations of photosynthetic accessory proteins-the Phycobilisome Isolation. Phycobilisomes were isolated as phycobiliproteins. These proteins are assembled into particles described (3). In some experiments, Triton was replaced by the (phycobilisomes) that are attached in regular arrays to the outer zwitterionic detergent Deriphat 160 (gift of General Mills surface of the thylakoids (1). The polypeptide composition of Chemical Division, Kankakee, IL) or Miranol S2M-SF (gift of phycobilisomes is complex. In addition to multiple copies of the Miranol Chemical Corp., Irvington, NJ). In these experiments, subunits of the phycobiliproteins, these particles contain detergent was used at a final concentration of 1-1.5% (wt/vol) polypeptides believed to function in the assembly of the chro- to solubilize phycobilisomes from membrane particles in broken moproteins and in the attachment of the particles to the cell suspensions; it was also present in the sucrose gradients at membrane (2, 3). Preparation of phycobilisomes with unim- 0.1%. paired energy transfer properties has invariably involved Sedimentation Studies. Sedimentation coefficients were breakage of the cells in solutions of 0.65-0.75 M NaH2PO4 ti- determined with a Spinco model E analytical ultracentrifuge trated to pH 7.0-8.0 with K2HPO4 (NaK-PO4), incubation with equipped with absorption optics. Experiments were performed 1-2% Triton X-100, and isolation by centrifugation in density at protein concentrations between 0.13 and 1.5 mg/ml in 0.75 gradients of sucrose in the same buffer (4, 5). Triton X-100 is M NaK-PO4, using 12-mm double-sector cells in an AN-D rotor insoluble in this buffer and is totally removed from the phy- at speeds of 26,000-30,000 rpm. Corrections of sobs to s20,w were cobilisomes during sedimentation (4, 6). made based on the determination of relative density and vis- It is generally recognized that membrane proteins, isolated cosity values of 1.0988 and 1.3556, respectively, for the refer- with detergents, form aggregates upon removal of the deter- ence buffer, 0.75 M NaK-PO4. No correction was made for gents. Such aggregates are an artifact of the isolation procedure possible preferential hydration of the phycobilisomes; however, and do not represent structures present in the cell. The proce- such behavior is not unlikely in 0.75 M NaK-PO4 (see ref. 11). dure for phycobilisome preparation (here called the "Triton A partial specific volume of 0.73 ml/g was estimated for phy- procedure") may be anticipated to lead to the formation of cobilisomes from strain 6301, based on the known amino acid aggregates. Indeed, although phycobilisome preparations from compositions of phycocyanin (12), and allophycocyanin (13), certain cyanobacteria sediment as a single component in and the Mr 30,000 and-33,000 polypeptides (unpublished data). high-salt sucrose density gradients, those from other organisms The above components account for about 94% of the protein give either two bands or a continuous smear covering a mo- by weight in these phycobilisomes (3). lecular weight range of several million (ref. 7; unpublished Electron Microscopy. Copper grids (400 mesh) coated with observations). What, then, is the relationship between the a film of carbon over Formvar were used as specimen supports. aggregation state of phycobilisomes isolated by the Triton just before sample application they were exposed for about 5 procedure and that of particles isolated in the continuous sec to a 500 V/cm glow discharge in a partial vacuum (100 presence of detergent? To answer this question, we have pre- millitorrs). The phycobilisome specimens were diluted from pared phycobilisomes in two zwitterionic detergents that are their initial concentration of about 2 mg/ml to 20 ,g/ml im- readily soluble in the high-salt buffers. In such detergents, mediately prior to application to the support films in 5-iA drops. Samples isolated with the aid of Triton X-100 were diluted in The publication costs of this article were defrayed in part by page 0.75 M NaK-PO4 buffer. The diluent for those isolated in charge payment. This article must therefore be hereby marked "ad- vertisement" in accordance with 18 U. S. C. §1734 solely to indicate Abbreviation: NaK-PO4, NaH2PO4 titrated to pH 8.0 with this fact. K2HPO4. 6162 Downloaded by guest on September 29, 2021 Biochemistry: Glazer et al. Proc. Natl. Acad. Sci. USA 76 (1979) 6163 zwitterionic detergents contained, additionally, the appropriate emission and decrease in energy transfer to allophycocyanin detergent at 0.1%. After about 30-sec residence time, each 5-,ul and allophycocyanin B (3) when phycobilisome solutions are sample drop was almost entirely withdrawn by suction friom incubated at low protein concentrations (data not shown). a fine-tip pipet (14), and the residual liquid film was overlaid Attempts to isolate phycobilisomes from strain 6312 by the with a 5-,ul drop of glutaraldehyde that had been freshly diluted Triton procedure used with strain 6301 (3) resulted in the dis- to 0.3% in the buffer used for sample dilution. After 5 min, the tribution of phycobilisome material on sucrose density gradients grid was serially rinsed in 100 mM and 10 mM ammonium illustrated in tube A of Fig. 1 Inset. Two deep blue bands of acetate, care being taken to wet only the sample side of the grid. protein were seen against a continuum of blue material, ex- After removal of the 10 mM rinse solution, a 5-,Ml drop of 1-2% tending from a free phycobiliprotein fraction at the top to aqueous uranyl formate was applied, left on for several seconds, nearly the bottom of the centrifuge tube. Samples from bands and thoroughly removed by suction applied to one edge of the 1 and 2 from such a gradient had virtually identical absorption grid. The uranyl formate stain was unbuffered (pH nz2.0) and spectra (Fig. 1, solid line). In the analytical ultracentrifuge, band had been passed through a 25-nm Millipore filter within a few 1 represented a single component of about 55 S. Band 2, how- hours of use. Each specimen was observed in a JEOL 100 B ever, gave rise to two distinct species: a small component of electron microscope immediately after preparation. Primary about 55 S, probably identical to that present in band 1, and a magnifications were X20,000-X45,000. All micrographs were large, more polydisperse component (as judged from the shape obtained by use of the technique of minimal beam exposure (15) of the boundary) with an average sedimentation coefficient of as adapted to the JEOL instrument. 120-130 S. In both of the experiments described in Table 1, the Other Methods. Density measurements were performed in smaller component in band 2 represented only about 10% of a Precision density meter (DMA 02C) and viscosity measure- the total material in the sample. ments were made in an Ostwald capillary viscometer. Gel Phycobilisome Aggregation During Isolation. In investi- electrophoresis and all other measurements were performed gating the question of phycobilisome aggregation during su- as described (3). crose gradient centrifugation, we tested two zwitterionic de- tergents, Miranol and Deriphat, in place of the nonionic de- RESULTS tergent Triton X-100. It has been shown recently that these The phycobilisomes of Synechococcus spp. 6301 and 6312 were detergents efficiently solubilize the protein-chlorophyll com- chosen for comparative study because they represent extremes plexes of photosynthetic membranes (17). Unlike Triton X-100, in the relative ratios of the major phycobiliproteins, C-phyco- which formed a floating detergent layer, the zwitterionic de- cyanin and allophycocyanin. The phycocyanin/allophyco- tergents were soluble in 0.75 M NaK-PO4 and could thus be cyanin ratios are -6:1 (3) and ;1.5:1 (16) in Synechococcus spp. added to sucrose gradients in high-salt buffer. Phycobilisomes 6301 and 6312, respectively. Neither of these phycobilisomes solubilized from cell extracts in such a system could be separated contains the red biliprotein phycoerythrin present in Syne- by sedimentation from lipids in the continuous presence of chocystis sp. 6701 in a weight ratio of ;2:2:1 to phycocyanin excess detergent, minimizing aggregation due to hydrophobic and allophycocyanin. interactions (see Discussion). Ultracentrifuge Studies. In sedimentation velocity experi- Experiments with strain 6301 showed no difference in the ments with Synechococcus sp. 6301 phycobilisomes, prepared phycobilisome profile on sucrose gradients when either Miranol by the Triton procedure, a single boundary with s20,W = 61-63 or Deriphat was used in place of Triton X-100. With strain 6312, S was obtained for several different samples in the protein however, these detergents produced significant changes in the concentration range 0.66-1.5 mg/ml. A significant decrease distribution of material seen on sucrose gradients. Miranol in S20,w was observed for a sample at a protein concentration produced a single, broad phycobilisome region, containing some of 0.13 mg/ml (Table 1). This decrease appears to be due to partial time-dependent dissociation of phycobilisomes with decreasing protein concentration. This interpretation is con- sistent with an observed increase in phycocyanin fluorescence

Table 1. S20,w values for phycobilisomes from Synechococcus spp. 6301 and 6312 Strain 6301 Strain 6312 Protein, Protein, mg/ml S20,w, S mg/ml S20,w, S

0.13 53.7 0.4 54.91 * 0.66 61.7 133 J band 2 0.70 63.0 0.3 55.5 band 1 0.84 61.2 0.4 54.61 band 2 0.84 61.2 120j 1.50 61.5 0.6 48.2t Phycobilisomes were isolated as described (3) and prepared for analytical ultracentrifugation by dialysis against 0.75 M NaK-PO4, Wavelength (nm) pH 8.0. FIG. 1. Absorption spectra of phycobilisomes from Synecho- * See Fig. 1 Inset. coccus sp. 6312, isolated from cell extracts solubilized with Triton t In this experiment, Deriphat 160 was used in place of Triton X-100 X-100 (-) or Deriphat 160 (---). (Inset) Sucrose step gradient as described in Materials and Methods. Deriphat 160 was also in- profiles after centrifugation of the soluble supernatant fractions from cluded in the sample and reference buffers during analytical ul- cell extracts solubilized with Triton X-100 (tube A) or Deriphat 160 tracentrifugation at a concentration of0.1%. No correction was made (tube B). The sucrose gradient shown in tube B contained 0.1% Der- for preferential binding of detergent or the effect of detergent on iphat 160. Numbers at the left represent initial sucrose concentrations. viscosity. Sedimentation coefficients are given in Table 1. Downloaded by guest on September 29, 2021 6164 Biochemistry: Glazer et al. Proc. Natl. Acad. Sci. USA 76 (1979)

chlorophyll a and sedimenting to approximately the same po- sorbance at Xmax (628 nm) and analyzed by difference spec- sition as the band 1 material shown in tube A (Fig. 1 Inset). The troscopy (Triton X-100 and Deriphat phycobilisomes in the chlorophyll a contamination could be avoided by inclusion of reference and sample cells, respectively), a distinct peak at 651 higher levels of Miranol in the gradient. A trace amount of nm, characteristic of allophycocyanin was present, with an macroscopic blue particles was found slightly below the band absorbance equivalent to about 4% of the sample absorbance 2 material in tube A. In the presence of Deriphat, a single blue at 628 nm. phycobilisome band was seen (tube B, Fig. 1 Inset), and the The fluorescence emission spectrum for strain 6312 phyco- sucrose gradient profile resembled that seen for phycobilisome bilisomes isolated with Deriphat had an emission maximum at preparations from strain 6301 (figure 1 of ref. 3). Such particles a slightly longer wavelength (670 nm) than the band 1 phyco- were homogeneous in the analytical ultracentrifuge and sedi- bilisomes from the Triton X-100 preparation (668 nm) (Fig. 3). mented with an appareflt S20,w of 48.2 S for the protein-de- These fluorescence data are consistent with a sflightly higher tergent complex (Table 1). allophycocyanin content in the Deriphat preparation. A notable Properties of Isolated Phycobilisomes. Phycobilisomes observation, illustrated in Fig. 3, is that the phycobilisomes from isolated with the aid of Miranol or Deriphat were analyzed by band 2 of the Triton X-100 preparation showed a much de- gel electrophoresis in the presence of sodium dodecyl sulfate creased yield of fluorescence. The aggregation of phycobili- and compared to phycobilisomes prepared by the Triton pro- somes evidently leads to quenching of the fluorescence. cedure. The polypeptide composition of phycobilisomes from Electron Micrographs. Electron micrographs of phycobil- either strain 6301 or strain 6312 was the same in preparations isomes from Synechocystis sp. 6701 and from Synechococcus obtained with the aid of all three detergents, with one minor spp. 6312 and 6301, are shown in Fig. 4. Particles isolated from exception (Fig. 2). An additional polypeptide of Mr 40,000 was Synechocystis sp. 6701 (Fig. 4A) are generally similar in shape present in very small amount in the band 1 material (Fig. 2, lane and size, although scarcely identical. At least as seen in their 4) from strain 6312 phycobilisomes prepared with Triton X-100 flattened-down aspect they have the approximate shape of a but was absent in the band 2 tnaterial and, in phycobilisomes hemi-disc. Two distinguishable substructures were seen: a tri- from Miranol and Deriphat preparations (lanes 5-7). angular "core" composed of three objects that could be spheres, The phycobilisomes from strain 6301 displayed similar ab- or discs seen in face view; and short rods composed of stacked sorption spectra, independent of the detergent used in the iso- discs whose number per rod depends upon the culture condi- lation. Strain 6312 phycobilisomes isolated in the presence of tions (7). The triangular array of the core is invariably oriented Deriphat exhibited an absorption spectrum with slightly higher so that two of its units lie along one edge of the phycobilisome absorbance at long wavelength than those prepared with Triton particle and the third unit is at the approximate center of the X-100. When such samples were normalized to equal ab- particle. The phycobilisomes from Synechococcus sp. 6312 have some 6301 6312

1.0

r §-0 A b 0.8

(A I i o~~ ~ ~ \

(0.7 4

0.2 aa

1 2 3 4 5 6 7 600 650 700 750 FIG. 2. Sodium dodecyl sulfate/polyacrylamide gel electropho- Wavelength (nm) resis of isolated phycobilisomes from Synechococcus sp. 6301 (lanes 1-3) and Synechococcus sp. 6312 (lanes 4-7). Samples in lanes 1, 4, FIG. 3. Corrected fluorescence emission spectra of phycobili- and 5 were isolated by the Triton procedure. Lanes 4 and 5 represent somes from Synechococcus sp. 6312. All samples were diluted into bands 1 and 2, respectively, of the sample shown in tube A of Fig. 1. 0.75 M NaK-P04 at pH 8.0 to a final absorbance of 0.1 at 628 nm. Samples in lanes 2 and 6 were isolated in the presence of Miranol Excitation-asatq58"m ain] exciationkd andl emissin monochromator S2M-SF, and samples in lanes 3 and 7 were isolated in the presence slits were set at 4.0-nm bandpass. Phycobilisomes: Triton X-100 (see of Deriphat 160. Fig. 1) band 1 (- - - -) and band 2 (--- -) or Deriphat 160 (-). Downloaded by guest on September 29, 2021 Biochemistry: Glazer et al. Proc. Natl. Acad. Sci. USA 76 (1979) 6165

A B

FIG. 4. Electron micrographs of phycobilisomes from cyanobacteria (A) Synechocystis sp. 6701; "cores" are denoted by long arrows and "rods" by short arrows (see text). (B) Synechococcus sp. 6312; arrows as in A. (C) Field showing several phycobi- lisome aggregates from Synechococcus sp. 6312 processed by the Triton procedure (see text). (D) Phycobilisomes as in C but isolated in Deriphat. (E) Synechococcus sp. 6301. (C and D, X85,000; others, X150,000.)

structural similarity to those of Synechocystis sp. 6701 but are In the three phycobilisomes examined, the core elements distinctly less ordered (Fig. 4B). They contain the triangular- were 11.5 nm in diameter, and the discs composing the rods had shaped core but the rods (if they can be called that) only occa- diameters of 12 nm and a center-to-center spacing of 6.5 sionally exhibit stacked-disc structure. They are evidently so nm. short that any regular array they may have had in solution has been lost during drying in negative stain. The structures shown DISCUSSION here are like those found by Bryant et al. (7). We have shown that phycobilisomes can form aggregates when Differences of particle aggregation, resulting from isolating isolated in a high-salt medium that contains no detergent. This the phycobilisomes of Synechococcus sp. 6312 in the absence behavior is not surprising in such membrane-bound particles, or presence of a detergent (Deriphat), are shown in Fig. 4 C and believed generally to contain both hydrophobic and hydrophilic D. The former micrograph contains large particles estimated domains. Aggregative phenomena have been particularly to be aggregates of three to six unit-size phycobilisomes, a result well-studied in the case of penicillinase from B. licheniformis of preparation by the Triton procedure. The latter micrograph, (6) and the spike protein from Semliki Forest virus (18), both of a preparation containing 0.1% Deriphat, shows several of which exist in solution as small, uniform protein-detergent phycobilisome particles of standard, unit-size, with no sign of micelles in the presence of Triton X-100. When the detergent aggregation. was removed by sedimentation of the solution through deter- In the phycobilisomes from Synechococcus sp. 6301 (Fig. gent-free sucrose gradients, the molecules aggregated exten- 4E), the rods are distinctly longer than those found in sp. 6312 sively and irreversibly, and the aggregates retained both solu- and frequently contain three or more discs in stacked array. bility and uniformity of size (6). Behavior analogous to the There seem to be six rods in a well-preserved particle, but this above has been found in the phycobilisomes of Synechococcus estimate is uncertain because of the considerable variation seen. sp. 6312 (and in Fremyella diplosiphon and Anabaena varia- The core is unlike that of Synechocystis sp. 6701, Synecho- bilis, not reported here) but not in Synechococcus sp. 6301 or coccus sp. 6312, and other reported phycobilisomes (7, 8) in that Synechocystis sp. 6701 (and Porphyridium aerugineum, not it appears to contain only two units. It is clear that the ratio of reported here) with which the presence or absence of detergent the mass of rod material to core material is far greater in these in the sucrose gradients appears to be immaterial. phycobilisomes than in those of Synechococcus sp. 6312. The finding of aggregation in the absence of detergent for Downloaded by guest on September 29, 2021 6166 Biochemistry: Glazer et al. Proc. Natl. Acad. Sci. USA 76 (1979) the phycobilisomes of more than one organism is sobering, in- in which the molar ratio of phycoerythrin plus phycocyanin asmuch as standard preparative procedures have not previously to allophycocyanin is 4:1, and in Synechococcus sp. 6301, in reflected the fact that the detergent used, Triton X-100, is in- which the molar ratio of phycocyanin to allophycocyanin is soluble in the buffers used. Two questions arise from our ex- roughly 6:1 (3), the rods are long (Fig. 4 A and B). In Syne- perimental results. (i) Do the aggregates consist of multiple chococcus sp. 6312, in which the extraordinarily high content copies of the "monomers," or are they different in ways other of allophycocyanin is reflected in a phycocyanin-to-allophy- than size? (ii) In those cases in which there is no sedimentation cocyanin ratio of 1.5:1 (16), single discs are frequently seen to evidence for a spectrum of aggregation (e.g., detergent-in- be associated with the core, and rods longer than two discs are sensitive phycobilisomes, or when the detergent, Deriphat, has not seen. The above correlation is consistent with one of the been present), can it be ruled out that the particles actually main features of the models proposed for phycobilisomes-i.e., consist of aggregates of uniform size much like the aggregated that allophycocyanin is a major component of the core whereas spike protein of Semliki Forest virus (6, 18)? phycoerythrin and phycocyanin are in the rods (7, 8). The former question has an answer in the polypeptide maps and in the electron micrographs. For the Synechococcus sp. We thank Drs. Donald A. Bryant and Germaine Cohen-Bazire for 6312 phycobilisomes, the polypeptide composition is identical providing us with their manuscript on the structure of cyanobacterial whether a detergent is present or not. Thus, it is unlikely that phycobilisomes in advance of publication. We are indebted to Mr. J. aggregation has been brought about, or accompanied by, the C. Gingrich for the preparation of Synechocystis sp. 6701 phycobili- binding of adventitious polypeptides. The electron micrographs somes, and Mr. Victor Wong for darkroom assistance. This work was (for example, Fig. 4C) indicate strongly that the aggregates are supported in part by National Science Foundation Grants PCM 76- just that: collections of smaller, quite similar units. 15243A02 (to A.N.G.), PCM 77-01151 (to R.C.W.), and PCM 76-23308 The answer to the second question seems to be positive: for (to H.K.S.). each species of phycobilisome, the smallest particle found in abundance is the fundamental one, not an aggregate. The 1. Gantt, E. (1975) Bioscience 25,781-788. sedimentation coefficient of Synechococcus sp. 6301 phyco- 2. Tandeau de Marsac, N. & Cohen-Bazire, G. (1977) Proc. Natl. bilisomes and the smallest coefficients found for those of sp. Acad. Sci. USA 74, 1635-1639. 3. Yamanaka, G., Glazer, A. N. & Williams, R. C. (1978) J. Biol. 6312 are compatible with the size of the particles shown in Fig. Chem. 253,8303-8310. 4E (sp. 6301), with all the particles in Fig. 4D (sp. 6312, Deri- 4. Gantt, E. & Lipschultz, C. A. (1972) J. Cell Biol. 54, 313-324. phat), and with the smaller particles in Fig. 4C (sp. 6312, no 5. Gantt, E., Lipschultz, C. A., Grabowski, J. & Zimmerman, B. K. detergent). The detailed appearance of all the particles seen in (1979) Plant Physiol. 63,615-620. the electron micrographs (except the aggregated ones in Fig. 6. Simons, K., Helenius, A., Leonard, K., Sarvas, M. & Gething, M. 4C) suggests strongly that they are, structurally, the phycobi- J. (1978) Proc. Natl. Acad. Sci. USA 75,5306-5310. lisome monomer and are not aggregated, multiple copies of a 7. Bryant, D. A., Guglielmi, G., Tandeau de Marsac, N., Castets, smaller unit. A.-M. & Cohen-Bazire, G. (1979) Arch. Microbiol., in press. For studies of the physical characteristics of phycobilisomes, 8. Morschel, E., Koller, K. P., Wehrmeyer, W. & Schneider, H. their particle weights and particle structure, a detergent had (1977) Cytobiologie 16, 118-129. best be used the entire A nonionic 9. Rippka, R., Deruelles, J., Waterbury, J. B., Herdman, M. & throughout preparation. Stanier, R. Y. (1979) J. Gen. Microbiol. 111, 1-61. detergent, such as Triton X-100, is wholly inadequate, but a 10. Stanier, R. Y., Kunisawa, R., Mandel, M. & Cohen-Bazire, G. zwitterionic detergent, such as Deriphat, is soluble at all salt (1971) Bacteriol. Rev. 35, 171-205. concentrations needed. For purposes of electron microscopy 11. Kuntz, I. D., Jr. & Kauzmann, W. (1974) Adv. Protein Chem. 28, the availability of nonaggregated particles is an advantage. The 239-345. overall appearance of phycobilisomes of Synechocystis sp. 6701 12. Glazer, A. N. & Fang, S. (1973) J. Biol. Chem. 248, 659-662. is very similar to that recently reported (7); details of the ul- 13. Cohen-Bazire, G., Beguin, S., Rimon, S., Glazer, A. N. & Brown, trastructure of these particles and of mutants will be reported D. M. (1977) Arch. Microbiol. 111, 225-238. elsewhere. The core of the phycobilisomes of Synechococcus 14. Williams, R. C. (1977) Proc. Natl. Acad. Sci. USA 74, 2311- sp. 6312 appears very similar to that from Synechocystis sp. 2315. 6701-a triangular array of spheres or discs in axial view. But 15. Williams, R. C. & Fisher, H. W. (1970) J. Mol. Biol. 52, 121- it to 123. is be noted that the phycobilisomes of Synechococcus sp. 16. Lemasson, C., Tandeau de Marsac, N. & Cohen-Bazire, G. (1973) 6301 contain only two disc-shaped objects in their cores. They Proc. Natl. Acad. Sci. USA 70,3130-3133. are evidently built on lines different from those recently de- 17. Markwell, J. P., Miles, C. D., Boggs, R. T. & Thornber, J. P. (1979) scribed in models (7, 8). FEBS Lett. 99, 11-14. The length of the rods depends upon the phycobiliprotein 18. Simons, K., Helenius, A. & Garoff, H. (1973) J. Mol. Biol. 80, composition of the phycobilisomes. In Synechocystis sp. 6701, 119-133. Downloaded by guest on September 29, 2021