ALLOPHYCOCYANIN FORMS ISOLATED FROM NOSTOC SP. PHYCOBILISOMES*

Abstract

Allophycocyanin from dissociated phycobilisomes of Nostoc sp. occurs in three spectrally identifiable forms that fractionate on calcium phosphate adsorption chromatography as: allophycocyanin (APC) I (15-20%), APC I1 (4&50%), and APC I11 (3@40%). APC I has a single absorption maximum at 654 nm, and a fluorescence emission peak at 678 nm. The absorption peaks of APC I1 and 111 are both at 650nm, but the relative absorbance at 620/650nm of APCIII is less than that of APCII. The emission of both is maximum at 660nm. On zone sedimentation in sucrose, their S,,,, values of 6.0 0.2 (APC 111) were comparable to the order of their elution from Sephadex G-200. On SDS acrylamide gel electrophoresis two subunits were resolved with apparent molecular weights of 16,900 and 18,400 daltons. When stained by Coomassie blue, they were present in a ratio of 1a:lP in APC11 and 111, and a probable ratio of 2a:3P in APCI. The larger size of APCI may be accounted for by additional / 3 subunits, by the presence of an additional polypeptide of 35,000 daltons, or both. Over several days, bleaching as noted by a decrease in absorbance at 650nm, occurred in all three forms; in addition, the more pronounced bleaching at 650 nm, relative to 620 nm, results in APC 111 becoming spectrally identical to APC 11. A trace of a fourth pigment, probably comparable to allophycocyanin-B, was occasionally detected. The results suggest that several in vitro APC forms (sharing similar subunits) arise upon phycobilisome dissociation, and that APC I is the form most closely related to the final fluorescence emitter of intact phycobilisomes. In this form it probably serves as the bridging pigment in energy transfer from the phycobilisomes to chlorophyll. 0.1 (APC I), 5.0 & 0.1 (APC 11), and 5.3 INTRODUCTIONLight energy absorbed by phycobiliproteins is first funneled to photosystem 11, as recently reported for Porphyridiurn cruentum (Ley and Butler, 1976), and is finally distributed t o both photosystems, as for Anacystis nidulans, where 40% of the quanta absorbed by phycocyanin are contributed t o photosystem I and 60% t o photosystem I1 (Wang et a/., 1977). Phycobilisomes (PBsomes)?, which contain the phycobiliproteins, have allophycocyanin (APC) as the key pigment (Gantt et al., , 1977. Its position is analogous to that of the trap of the two photosystems, but rather than doing photochemistry, it collects excitation energy from the other biliproteins before funnelling it into the chlorophyll of the thylakoid. Phycobilisomes, isolated intact but stripped away from the chlorophyll-containing membranes, show a fluorescence maximum a t 6 7 5 4 8 0 n m (23"C), irrespective

Phycobilisomes from Blue-Green and Red Algae

“…One of these, designated as APC-B, nditions) 50 K-phosphate has been isolated from various blue-green and red algae (9, 13, nditions: 50 ma K-phosphate 18), another form designated as APC-I has thus far been purified It 545 nm made at 23 C. Figure from one blue-green alga (24) was: PE first, followed by R-PC, and finally by APC. The basis ¢.for the concept of uncoupling stages (under "Results") includes changes observed in isolated PC forms (I1) and the PE:PC complex (7) where the initial longer-wavelength-absorbing (627-637 nm) and -emitting (650-653 nm) forms gradually revert to shorterwavelength-absorbing (615-623 nm) and -emitting (640-643 nm) forms.…”

Isolation and Characterization of the Central Component of the Phycobilisome Core of Nostoc sp.

“…isolated from the Chl-containing membranes fluoresce at 678 nm (20). This corresponds to the fluorescence emission maximum oftwo specialized APC molecules isolated from this organism, called APC I and APC B (16,20). Therefore, excitation energy appears to be transferred to APC I and/or B from other biliproteins and then from these specialized APC forms to Chl a in the thylakoids.…”

“…isolated from the Chl-containing membranes fluoresce at 678 nm (20). This corresponds to the fluorescence emission maximum oftwo specialized APC molecules isolated from this organism, called APC I and APC B (16,20).…”

Phycobilisome-thylakoid Topography on Photosynthetically Active Vesicles of Porphyridium cruentum

“…In several cyanobacteria APC-I has a similar long wavelength emission (4, 'Abbreviations: PBS, phycobilisome(s); APC, allophycocyanins; PBS-vesicles, thylakoid vesicles with attached phycobilisomes; PE, phycoerythrin; SPC, sucrose-phosphate-citrate medium. 18,25) and can serve as a terminal pigment as well. In Anabaena variabilis, isolated vesicles with bound PBS exhibited a decrease of the F695 nm relative to F685 nm with decreasing 02 evolution in degenerating conditions (14).…”