The molecular basis of nuclear genetic code change in ciliates.

Abstract

The nuclear genetic code has changed in several lineages of ciliates. These changes, UAR to glutamine and UGA to cysteine, imply that eukaryotic release factor 1 (eRF1), the protein that recognizes stop codons and terminates translation, changes specificity. Here we test whether changes in eRF1 drive genetic code evolution. Database sequence analysis reveals numerous genetic code alterations in ciliates, including UGA --> tryptophan in Blepharisma americanum and the distantly related Colpoda. We sequenced eRF1 from four ciliates: B. americanum, a heterotrich that independently derived the same eRF1 specificity as Euplotes, and three spirotrichs, Stylonychia lemnae, S. mytilus, and Oxytricha trifallax, that independently derived the same genetic code as Tetrahymena (UAR --> glutamine). Distantly related ciliates with similar codes show characteristic changes in eRF1. We used a sliding window analysis to test associations between changes in specific eRF1 residues and changes in the genetic code. The regions of eRF1 that display convergent substitutions are identical to those identified in a recently reported nonsense suppression mutant screen in yeast. Genetic code change by stop codon reassignment is surprisingly frequent in ciliates, with UGA --> tryptophan occurring twice independently. This is the first description of this code, previously found only in bacteria and mitochondria, in a eukaryotic nuclear genome. eRF1 has evolved strikingly convergently in lineages with variant genetic codes. The strong concordance with biochemical data indicates that our methodology may be generally useful for detecting molecular determinants of biochemical changes in evolution.