ISOLATION AND CHARACTERIZATION OF PHOTOSYSTEM-I AND PHOTOSYSTEM-II FROM THE RED ALGA PORPHYRIDIUM-CRUENTUM

Photosystem I and Photosystem II complexes were isolated from the red alga Porphyridium cruentum (P. purpureum) following solubilization of thylakoid membranes with dodecyl-B-D-maltoside. More than 90% of the t otal chlorophyll was recovered in two green bands on a sucrose gradien t. PS I and PS II complexes, present in the lower green band, were sep arated by anion exchange chromatography. The purified PS II complex, w ith an absorption maximum at 674 nm and a fluorescence emission maximu m at 699 nm (77 K), contained the reaction center polypeptide D2 and t he core antenna proteins CP43 and CP47 as shown by immunoblot analysis . Two bands (<14 kDa), presumably the cyt b-559 subunits were also obs erved after silver-staining SDS-PAGE gels. The photoactivity rate of 5 8 mu mol DCPIP/mg Chi per h with DPC for PS II core complexes, and 112 mu mol DCPIP/mg Chi per h for thylakoids (minus phycobilisomes), was sensitive to DCMU inhibition. The purified PS I holocomplex had approx 130 chlorophyll per P700, an absorption maximum at 680 nm and a fluor escence emission maximum at 730 nm (at 77 K). Prominent silver-stained bands in SDS-PAGE gels at 62 and 57 kDa polypeptides were immunologic ally identified as the P700 apoproteins. Polypeptides of 23.5, 23, 22, 20, 19.5, 18, 16.5, 15.5, and 11 kDa and at least three low molecular mass polypeptides (< II kDa) were also observed. Of particular intere st are the 18-23.5 kDa polypeptides in the PS I complex, since such po lypeptides were previously unknown in phycobilisome-containing organis ms. In fact, we have found that the PS I complex of P. cruentum is str ucturally similar to that of higher plants, especially since polypepti des of 18-23.5 kDa were shown to be immunologically related to LHC I o f higher plants (Wolfe, G. et al. (1994) Nature 367, 566-568). These r esults clearly suggest that the thylakoid structure of this red alga i s more complex than in cyanobacteria, which like rhodophytes, also hav e phycobilisomes as major antenna complexes.