Abstract

Pseudo-reversion of the high-CO₂ requiring phenotype of the NADH dehydrogenase type 1-impaired mutant of Synechocystis PCC6803, strain M55, by salt stress coincides with partial restoration of PSI-driven cyclic electron transfer. In M55, the complete family of D proteins (D1–D6) that are needed for electron transfer through the complex is lacking. Adaptation to salt stress requires de novo synthesis of full-length 47-kDa ferredoxin-NADP⁺ reductase (FNR). A mutant created in the M55 background, which only expresses truncated chloroplast 37-kDa FNR cannot adapt to salt stress and refrains from growth in low CO₂. A special feature of FNR in cyanobacteria is the relatively high molecular mass of 44–48 kDa. A positively charged extended N-terminal domain of the cyanobacterial enzyme defines the extra mass. The extension likely plays a key role in the salt-stress inducible enhancement of PSI-driven cyclic electron transfer, and in the pseudo-reversion of the high-CO₂ requiring phenotype of M55. Data acquired with several other cyanobacteria and the oxychlorobacterium Prochlorothrix hollandica contributed to the present hypothesis. It proposes that FNR is involved in regulation of inducible and transient PSI cyclic electron transfer in cyanobacteria via a thylakoid surface charge and conditional-proteolysis steered mechanism.