Isolation and structural characterization of monomeric and trimeric photosystem I complexes (P700.FA/FB and P700.FX) from the cyanobacterium Synechocystis PCC 6803.

An isolation procedure was developed for the cyanobacterium Synechocystis 6803 (and 6714) which yields both monomeric and trimeric photosystem I complexes (P700.FA/FB complexes) depleted of the stroma-exposed subunits PsaC, -D, and -E (P700.FX complexes). Analysis by high resolution gel electrophoresis in combination with immunoblotting and N-terminal sequencing reveals the selective and quantitative removal of PsaC, -D, and -E from the P700.FA/FB complex, containing PsaA, -B, -C, -D, -E, -F, -K, -L and at least two subunits < or = 4 kDa. Monomeric and trimeric P700.FX complexes show an identical subunit composition and an identical charge recombination half-time of 750 +/- 250 microseconds as determined by flash-induced absorption change measurements, reflecting the quantitative loss of iron-sulfur clusters FA/FB and the presence of cluster FX. The existence of a stable trimeric P700.FX complex enables a detailed structural analysis by electron microscopy with high resolution. Comparison of averaged top and side view projections of P700.FX and P700.FA/FB complexes show that the height of the complex is reduced by about 2.5-3.3 nm upon removal 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-dimensional crystal analysis recently published for trimeric PS I of Synechococcus elongatus (Krauss, N., Hinrichs, W., Witt, I., Fromme, P., Pritzkow, W., Dauter, Z., Betzel, C., Wilson, K. S., Witt, H. T., and Saenger, W. (1993) Nature 361, 326-330), only the structural analysis presented here is able to assign the stroma-exposed mass exclusively to the subunits PsaC, -D, and -E and to exclude a contribution of other subunits.