Antenna photosystem I remodeling in the iron stress–induced complex from Anabaena sp. PCC 7120

The structure and oligomeric states of the photosystem I–iron stress–induced protein A (PSI-IsiA) supercomplex from several species have been investigated using cryo–electron microscopy (cryo-EM) and atomic force microscopy. The cryo-EM structure of the PSI-IsiA complex from cyanobacterium Anabaena sp. PCC 7120 (7120) has been published previously. However, not all IsiA and PSI subunits were identified. In this work, we have structurally and spectroscopically characterized the PSI-IsiA supercomplex from 7120 using single-particle cryo-EM at a resolution of 2.2 angstroms. The data resolved all individual IsiA and PSI subunits, highlighting features that distinguish the 7120 PSI-IsiA from other complexes, including additional IsiA and a unique PsaK subunit specific for PSI-IsiA. Time-resolved fluorescence spectroscopy showed tight energetic coupling of IsiA to PSI, which may play a role in lowering antenna fluorescence yield and increasing the efficiency of energy transfer. This work extends the scope of PSI cellular remodeling in response to different stress conditions.