Structure and organization of phycobilisomes on membranes of the red alga Porphyridium cruentum

Abstract

In the present work, electron microscopy and single particle averaging was performed to investigate the supramolecular architecture of hemiellipsoidal phycobilisomes from the unicellular red alga Porphyridium cruentum. The dimensions were measured as 60 x 41 x 34 nm (length x width x height) for randomly ordered phycobilisomes, seen under high-light conditions. The hemiellipsoidal phycobilisomes were found to have a relatively flexible conformation. In closely packed semi-crystalline arrays, observed under low-light conditions, the width is reduced to 31 or 35 nm, about twice the width of the phycobilisome of the cyanobacterium Synechocystis sp. PCC 6803. Since the latter size matches the width of dimeric PSII, we suggest that one PBS lines up with one PSII dimer in cyanobacteria. In red algae, a similar 1:1 ratio under low-light conditions may indicate that the red algal phycobilisome is enlarged by a membrane-bound peripheral antenna which is absent in cyanobacteria.

Fig. 1

Typical examples of negatively stained cyanobacterial membranes and associated phycobilisomes showing multiple ordered 2-D arrays under low-light conditions (left frame) and a random distributiom under high-light conditions (right frame) and the space bar equals 1 μm for the left frame and 400 nm for the right frame

Fig. 2

Single particle electron microscopy of phycobilisome projections from Porphyridium cruentum (A) View in the membrane plane of membrane-bound phycobilisomes (B) View in the membrane plane of free phycobilisomes (C–E) side views of free phycobilisomes. (F) Most common phycobilisome fragment. (G) image of A with 2-fold rotational symmetry imposed after analysis (H) Image of C, with mirror symmetry imposed after analysis. The number of summed projections for A-F is 1520, 484, 4096, 380, 499 and 746, respectively. The space bar is 25 nm

Fig. 3

Comparison of semi-crystalline arrays of phycobilisomes. (A) the membrane-attached phycobilisome (B) APC core structure of Synechocystis PCC 6803 ck mutant, sum of 5019 projections for comparison (A.A. Arteni, R. Ajlani, E.J. Boekema, unpublished results). (C) 2D array with semi-crystalline phycobilisomes, an average of 11 fragments (D) 2D array with semi-crystalline phycobilisomes, an average of 107 fragments (E) PSII dimer (from Thermosynechococcus elongatus) (F) PSII double dimer, found after solubilization of membranes with dodecyl maltoside, a sum of 59 projections (G) PSII double dimer, found after solubilization of membranes with digitonin, a sum of 46 projections. The space bar is 50 nm