Photosystem I structural fusion with IsiA enhances light-harvesting capabilities of filamentous cyanobacterium during iron deficiency.

During iron stress, cyanobacteria produce iron-stress-inducible protein A (IsiA), which associates with photosystem I (PSI) to support cell growth. In most cyanobacteria, the stress induces formation of large IsiA18-PSI3 complexes. However, some species, including Anabaena sp. PCC 7120, studied in this work, form an unusual, smaller PSI-monomer-based IsiA-PSI supercomplex containing a unique fusion of IsiA with the PSI subunit PsaL, IsiA-PsaL. In this study, genetically tagged IsiA-PsaL was used to isolate several IsiA-PsaL-containing supercomplexes composed of multiple IsiA homologs. A combination of static and time-resolved optical spectroscopies was used to evaluate the energetic properties of gel-embedded supercomplexes, including the excitation energy transfer efficiency from IsiA-PsaL to PSI. These analyses demonstrated that excitation energy transfer in the IsiA-PsaL complex is highly efficient. Enhanced efficiency may be a key factor underlying the downsizing of the IsiA-PSI assembly, which could compensate for the smaller size of the complex.