A photosystem 1 psaFJ-null mutant of the cyanobacterium Synechocystis PCC 6803 expresses the isiAB operon under iron replete conditions.

A psaFJ-null mutant of Synechocystis sp. strain PCC 6803 was characterised. As opposed to similar mutants in chloroplasts of green algae, electron transfer from plastocyanin to photosystem 1 was not affected. Instead, a restraint in full chain photosynthetic electron transfer was correlated to malfunction of photosystem 1 at its stromal side. Our hypothesis is that absence of PsaF causes oxidative stress, which triggers the induction of the 'iron stress inducible' operon isiAB. Products are the IsiA chlorophyll-binding protein (CP43') and the isiB gene product flavodoxin. Supporting evidence was obtained by similar isiAB induction in wild type cells artificially exposed to oxidative stress.