Photosystem I of the thermophilic cyanobacterium Chroococcidiopsis sp. TS-821 forms tetramers composed of a dimer of dimers, but can also form classical trimers.

A cryo-EM 3D reconstruction of the PSI tetramer complex shows details at ~ 11.5 Å resolution.

Comparison of the EM model with the structure of PSI of Thermosynechococcus elongatus reveals key differences in which subunits are involved in the two different interfaces, interface type 1 within a dimer and interface type 2 between dimers.

The type 1 interface in TS-821 is similar to the monomer interface in the trimeric PSI from T. elongatus, with interactions between subunits PsaA, -B, -I, -L and M. In type 2 the interaction is only between PsaA, -B and –L.

The physiological or evolutionary advantage of the tetramer is unknown. However, the presence of both dimers and tetramers in the membrane suggest a dynamic equilibrium that shifts towards the tetramers in high light.