A non-canonical genetic code in an early diverging eukaryotic lineage

The nearly invariant nature of the 'Universal Genetic Code' attests to its early establishment in evolution and to the difficulty of altering it now, since so many molecules are required for, and depend upon, faithful translation. Nevertheless, variations on the universal code are known in a handful of genomes. We have found one such variant in diplomonads, an early-diverging eukaryotic lineage. Genes for alpha-tubulin, beta-tubulin and elongation factor 1 alpha (EF-1alpha) from two unclassified strains of Hexamitidae were found to contain TAA and TAG (TAR) triplets at positions suggesting a variant code in which TAR codes for glutamine. We found confirmation of this hypothesis by identifying genes encoding glutamine-tRNAs with CUA and UUA anticodons. The alpha-tubulin and EF-1alpha genes from two other diplomonads, Spironucleus muris and Hexamita inflata, were also sequenced and shown to contain no such non-canonical codons. However, tRNA genes with the anticodons UUA and CUA were found in H.inflata, suggesting that this diplomonad also uses these codons, albeit infrequently. The high GC content of these genomes and the presence of two isoaccepting tRNAs compound the difficulty of understanding how this variant code arose by strictly neutral means.