Cyanobacteria are a diverse group of prokaryotes and the only prokaryotes capable of oxygenic photosynthesis. They exist in a variety of environments and play crucial roles in the global carbon and nitrogen cycles. Synechocystis sp. PCC 6803 (Synechocystis) has been used extensively as a model cyanobacterium for studies concerned with photosynthesis and environmental adaptation.
Lysine methylation is a conserved and dynamic regulatory post-translational modification performed by lysine methyltransferases (KMTs). Although accumulating evidence suggests that KMTs provide a mechanism involved in the regulation of metabolism and cellular physiology in organisms ranging from bacteria to mammals, the extent and function of KMT in photosynthetic organisms remain largely unexplored.
In a recent study published in Molecular & Cellular Proteomics, the research group led by Prof. GE Feng from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences has revealed the novel molecular mechanisms of lysine methyltransferases cyanobacterial lysine methyltransferase 1 (cKMT1).
The researchers first tested all of the putative KMTs in the Synechocystis, and determined that cKMT1 can catalyze lysine methylation both in vivo and in vitro. The lack of cKMT1 impaired both cyclic electron transport and state transition in Synechocystis.
Further quantitative methylation analysis between ΔcKMT1 and wild-type (WT) cells identified 305 class I lysine methylation sites in 232 proteins, with 80 methylation sites in 58 proteins hypomethylated in ΔcKMT1 cells. Methylation assay revealed that cKMT1 could bind to ferredoxin-NADP(+) reductase (FNR) and catalyze its methylation on K139 and K208.
According to the structure simulation, site-directed mutagenesis, and KMT activity measurement, H118 and Y219 of cKMT1 were identified as key residues in mediating the binding of S-adenosyl-L-methionine (SAM).
The researchers used mutations that mimicked the unmethylated forms of FNR to show that K139 was a key site of FNR, and that at least one physiological function of FNR methylation at key lysine residues was to regulate energy transfer in Synechocystis.
The findings of this study identified a new KMT in Synechocystis, and elucidated a methylation-mediated molecular mechanism catalyzed by cKMT1 for the regulation of energy transfer and state transition in cyanobacteria.