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

Abstract

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.