ISOLATION AND STRUCTURAL CHARACTERIZATION OF MONOMERIC AND TRIMERIC PHOTOSYSTEM-I COMPLEXES (P700-CENTER-DOT-FA FB AND P700-CENTER-DOT-FX) FROM THE CYANOBACTERIUM SYNECHOCYSTIS PCC-6803/

An isolation procedure was developed for the cyanobacterium Synechocys tis 6803 (and 6714) which yields both monomeric and trimeric photosyst em I complexes (P700.F(A)/F(B) complexes) depleted of the stroma-expos ed subunits PsaC, -D, and -E (P700-F(x) complexes). Analysis by high r esolution gel electrophoresis in combination with immunoblotting and N -terminal sequencing reveals the selective and quantitative removal of PsaC, -D, and -E from the P700-F(A)/F(B) complex, containing PsaA, -B , -C, -D, -E, -F, -K, -L and at least two subunits less-than-or-equal- to 4 kDa. Monomeric and trimeric P700-F(x) complexes show an identical subunit composition and an identical charge recombination half-time o f 750 +/- 250 mus as determined by flash-induced absorption change mea surements, reflecting the quantitative loss of iron-sulfur clusters F( A)/F(B) and the presence of cluster F(x). The existence of a stable tr imeric P700.F(x) complex enables a detailed structural analysis by ele ctron microscopy with high resolution. Comparison of averaged top and side view projections of P700.F(x) and P700.F(A)/F(B) complexes show t hat the height of the complex is reduced by about 2.5-3.3 nm upon remo val of the three stroma-exposed subunits and indicate the position of these three subunits on the PS I surface. While the outer contours of the stroma exposed mass of PS I agree very well. with the three-dimens ional crystal analysis recently published for trimeric PS I of Synecho coccus elongatus (Krauss, N., Hinrichs, W., Witt, I., Fromme, P., Prit zkow, W., Dauter, Z., Betzel, C., Wilson, K. S., Witt, H. T., and Saen ger, W. (1993) Nature 361, 326-330), only the structural analysis pres ented here is able to assign the stroma-exposed mass exclusively to th e subunits PsaC, -D, and -E and to exclude a contribution of other sub units.