The trimeric organisation of photosystem I is not necessary for the iron-stress induced CP43' protein to functionally associate with this reaction centre.

A mutant of Synechocystis PCC 6803 lacking the PsaL subunit of photosystem I (PSI) has been grown in iron-deficient media to induce the expression of the isiA gene, which encodes the chlorophyll a-binding protein CP43'. The purpose of this was to establish whether or not the formation of an 18-mer CP43'-PSI supercomplex reported for wild type Synechocystis cells [Nature 412 (2001) 743-745] was dependent on the trimeric conformation of the cyanobacterial PSI reaction centre. Structural characterisation by electron microscopy and single particle image analysis has revealed that the PsaL-mutant does not form trimers of PSI. However, despite this, CP43' was found to associate with the PSI monomer. The PSI monomer bound six or seven copies of CP43' along one edge of the PSI monomer and can be compared with one segment of the trimeric 18-mer CP43'-PSI supercomplex. We therefore conclude that the trimeric nature of cyanobacterial PSI is not required for the assembly of the CP43' antenna system under iron-deficient conditions.