Nucleic Acids Research, Vol 26, Issue 20 4557-4565, Copyright © 1998 by Oxford University Press
M Grimm, C Brunen-Nieweler, V Junker, K Heckmann and H Beier
Deviations from the universal genetic code have evolved independently
several times in ciliated protozoa. Thus, in some species UAA and UAG are
no longer used as termination codons, but are read as glutamine, whereas in
the genus Euplotes , UGA is translated as cysteine. We have investigated
the nature of the tRNACysisoacceptor responsible for decoding UGA in
Euplotes cells. Southern hybridization analyses indicated that a single DNA
molecule of 630 bp encoding tRNACys exists in the macronucleus of Euplotes
octocarinatus . Cloning and sequencing of this fragment revealed that it
contains only one copy of a tRNACys gene, which codes for a normal tRNACys
with GCA anticodon. This is the first report of the characterization of a
tRNA gene in any hypotrichous ciliate. It contains putative signals for
initiation and termination of transcription by RNA polymerase III and can
be transcribed efficiently in vitro in HeLa cell nuclear extract. Intensive
studies on the DNA and tRNA level involving PCR analyses have not disclosed
the existence of any tRNA Cys isoacceptor with UCA or ICA anticodons.
Translation of the UGA codon by t R N A sub G C A sup C y s necessitates a
G:A mispairing in the first anticodon position. We discuss a number of
aspects which might contribute to the finding that a near-cognate tRNA
isoacceptor efficiently translates the UGA stop codon.
ARTICLES
The hypotrichous ciliate Euplotes octocarinatus has only one type of tRNACys with GCA anticodon encoded on a single macronuclear DNA molecule
Institut fur Biochemie, Bayerische Julius-Maximilians-Universitat, Biozentrum, Am Hubland, D-97074 Wurzburg, Germany.
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