Evolution of Selenophosphate Synthetase

Abstract

Selenophosphate synthetase (SPS or SelD) provides the active selenium (Se) donor for the synthesis of selenocysteine (Sec), the 21st amino acid in the genetic code. In this chapter we summarize the distribution, phylogeny and function of all SPS genes across the tree of life. SPS is a selenoprotein itself in many prokaryotes (SelD) and eukaryotes (SPS2). As most other selenoproteins, SPS has orthologs with cysteine (Cys) in place of Sec. Although absent in many lineages, selenoproteins and SPS occur in bacteria, archaea and eukaryotes. In prokaryotes, SPS supports additional forms of Se utilization besides Sec, most notably the use of selenouridine in tRNAs. The study of selenophosphate synthetases, while serving a map of Se utilization across all sequenced organisms, also highlighted examples of functional diversification within this family. Within archaea, a few Crenarchaeota species exhibit a SelD-like gene. This is derived from SPS, but probably carries a different function, since it never co-occurs with other Se utilization genes. Within eukaryotes, many metazoan genomes, including humans, carry a paralog called SPS1 in addition to SPS2, which replaces the Sec/Cys site with some other amino acid (i.e., threonine, arginine, glycine, or leucine). Strikingly, SPS1 genes were generated through distinct gene duplication events of SPS2 in several metazoan clades (e.g., vertebrates and insects). Their function is still unknown, but it appears to be different from that of SPS2. Despite independently originated in parallel lineages, SPS1 genes were shown to share a common function. Thus, this function was likely already present in the parental SPS2 gene, driving the gene duplication events by sub-functionalization.