Molecular Biology and Evolution, Vol 14, 895-901, Copyright © 1997 by Society for Molecular Biology and Evolution
PJ Keeling and WF Doolittle
Recently, a group of diplomonads has been found to use a genetic code in
which TAA and TAG encode glutamine rather than termination. To survey the
distribution of this characteristic in diplomonads, we sought to identify
TAA and TAG codons at positions where glutamine is expected in genes for
alpha-tubulin, elongation factor-1 alpha, and the gamma subunit of
eukaryotic translation initiation factor-2. These sequences show that the
variant genetic code is utilized by almost all diplomonads, with the genus
Giardia alone using the universal genetic code. Comparative phylogenetic
analysis reveals that the switch to this genetic code took place very early
in the evolution of diplomonads and was likely a single event. Termination
signals and downstream untranslated regions were also cloned from three
Hexamita genes. In all three of these genes, the predicted TGA termination
codon was found at the expected position. Interestingly, the untranslated
regions of these genes are high in AT. This is incongruent with the coding
regions, which are comparatively GC-rich.
ORIGINAL ARTICLE
Widespread and ancient distribution of a noncanonical genetic code in diplomonads
Department of Biochemistry, Dalhousie University, Halifax, Nova Scotia.
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