Photosynthesis sustains life on Earth by converting carbon dioxide into organic compounds which can be utilized as an energy source by non-photosynthetic organisms. Photosynthetic light reactions produce NADPH and ATP, the energy currencies used for carbon assimilation. The production of NADPH is catalyzed by flavoenzymes known as ferredoxin-NADP+ oxidoreductases (FNRs), which function at the crossroads between the light reactions and carbon assimilation. All plant species contain multiple FNR enzymes, each having different substrate binding and catalytic properties. The different FNRs have traditionally been divided into two groups: photosynthetic leaf-type FNRs (LFNRs) and non-photosynthetic root-type FNRs (RFNRs). In addition to L- and RFNRs, plants also have flavoproteins that have not been functionally characterized. 

We studied the catalytic properties of an FNR homologue “FNR-LIKE” (FNRL), to investigate whether it can also function in the photoreduction of NADPH. We found that FNRL, in fact, resembles bacterial-type FNRs both structurally and biochemically. FNRL most likely interacts with one of the uncharacterized ferredoxins or ferredoxin-like proteins thus funneling electrons towards Fd-dependent metabolism. 

Arabidopsis thaliana (Arabidopsis) has two photosynthetic FNR isoforms, LFNR1 and LFNR2. Both proteins can be separated into acidic and basic forms with isoelectric focusing, suggesting that LFNRs are regulated via post-translational modification (PTM), which alters the isoelectric point of the enzymes. Because LFNRs are central metabolic enzymes, we have investigated the nature of this and other post-translational modifications of Arabidopsis LFNRs. We found that both LFNRs undergo N-terminal and lysine acetylation. Moreover, N-terminal acetylation was shown to be light responsive and to increase affinity towards ferredoxin. Our findings pave the way for further studies investigating the role of acetylation in the regulation of photosynthesis. 

Even though N-terminal and lysine acetylation have emerged as widespread modifications in chloroplasts during the recent years, the chloroplast acetylation machinery has remained mostly uncharacterized. Therefore, we set out to look for putative chloroplast acetyltransferases that could play a role in photosynthesis. We found that knock-out mutants of the acetyltransferase NUCLEAR SHUTTLE INTERACTING (NSI) were defective in state transitions, a regulatory mechanism that ensures excitation energy balance between photosystem I and II in low light. NSI was shown to acetylate lysine residues of several chloroplast proteins, including some involved in photosynthesis, suggesting that one or more of the acetylation sites regulated by NSI might be required for state transitions. Our results add another layer to the complex regulation of light harvesting in plants and will help further studies in unraveling the exact mechanism of state transitions.