Episodic evolution of a eukaryotic NADK repertoire of ancient provenance.

NAD kinase (NADK) is the sole enzyme that phosphorylates nicotinamide adenine dinucleotide (NAD+/NADH) into NADP+/NADPH, which provides the chemical reducing power in anabolic (biosynthetic) pathways. While prokaryotes typically encode a single NADK, eukaryotes encode multiple NADKs. How these different NADK genes are all related to each other and those of prokaryotes is not known. Here we conduct phylogenetic analysis of NADK genes and identify major clade-defining patterns of NADK evolution. First, almost all eukaryotic NADK genes belong to one of two ancient eukaryotic sister clades corresponding to cytosolic ("cyto") and mitochondrial ("mito") clades. Secondly, we find that the cyto-clade NADK gene is duplicated in connection with loss of the mito-clade NADK gene in several eukaryotic clades or with acquisition of plastids in Archaeplastida. Thirdly, we find that horizontal gene transfers from proteobacteria have replaced mitochondrial NADK genes in only a few rare cases. Last, we find that the eukaryotic cyto and mito paralogs are unrelated to independent duplications that occurred in sporulating bacteria, once in mycelial Actinobacteria and once in aerobic endospore-forming Firmicutes. Altogether these findings show that the eukaryotic NADK gene repertoire is ancient and evolves episodically with major evolutionary transitions.