Nucleic Acids Research, Vol 24, Issue 17 3424-3430, Copyright © 1996 by Oxford University Press
C Urban, K Zerfass, C Fingerhut and H Beier
We have recently identified chloroplast and cytoplasmic tRNACmCATrp as the
first natural UGA suppressor tRNAs in plants. The interaction of these
tRNAs with UGA involves a Cm: A mismatch at the first anticodon position.
We show here that tRNACmCATrp is incapable of misreading UAA and UAG codons
in vitro, implying that unconventional base pairs are not tolerated in the
middle anticodon position. Furthermore, we demonstrate that the ability of
tRNACmCATrp to promote UGA read-through depends on a quite simple codon
context. Part of the sequence surrounding the leaky UGA stop codon in
tobacco rattle virus RNA-1 was subcloned into a zein reporter gene and
read-through efficiency was measured by translation of RNA transcripts in
wheat germ extract. A number of mutations in the codons adjacent to the UGA
were introduced by site-directed mutagenesis. It was found that single
nucleotide exchanges at either side of the UGA had little effect on
read-through efficiency. A pronounced influence on suppression by
tRNACmCATrp was seen only if 2 or 3 nt at the 3'-side of the UGA codon had
been simultaneously replaced. As a consequence of the flexible codon
context accepted by tRNACmCATrp, this tRNA is able to misread the UGA in a
number of plant and animal viral RNAs that use translational read- through
for expression of some of their genes.
ARTICLES
UGA suppression by tRNACmCATrp occurs in diverse virus RNAs due to a limited influence of the codon context
Institut fur Biochemie, Bayerische Julius-Maximilians-Universitat, Wurzburg, Germany.
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