26 Recoding UGA as Selenocysteine

Abstract

THE VERSATILE UGA CODON
The UGA codon is more versatile than any other code word in the genetic alphabet. Best known as a termination codon, its primary function in most organisms, UGA also serves in several capacities as a sense codon. These include being decoded as tryptophan in mitochondria and mycoplasma, and as cysteine in some ciliates, due to divergence in the release factor and tRNA specificities in these organisms or organelle (for review, see Watanabe and Osawa 1995). At a small number of UGA codons in organisms whose primary use of UGA is to dictate cessation of protein synthesis, the default termination mode is apparently overridden by signals in the mRNAs, redirecting the translation machinery to incorporate an amino acid instead. UGA codons are the sites of +1 frameshifting in bacterial release factor 2 and eukaryotic antizyme, in both cases resulting in asparagine incorporation (for review, see Gesteland and Atkins 1996; Atkins et al. 1999). Examples of readthrough of UGA codons in the absence of frameshifting are known in phage Qβ, Sindbis virus, plant viruses, insects, and mammals, and may occur in all organisms. Of the numerous functions that UGA codons can assume, perhaps the most intriguing is “recoding” as selenocysteine. This usage is implemented in a wide variety of species in the eubacteria, archaea, and eukarya kingdoms.

Considerable knowledge has been gained about the process of selenocysteine incorporation in Escherichia coli, predominantly through elegant genetic and biochemical studies carried out in the Böck laboratory. The mechanism of selenocysteine...