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Mol. Cell. Biol., 04 1997, 2005-2013, Vol 17, No. 4
LE Hann, AC Webb, JM Cai and L Gehrke
We report that the competitive translational activity of alfalfa mosaic
virus coat protein mRNA (CP RNA), a nonadenylated mRNA, is determined in
part by the 3' untranslated region (UTR). Competitive translation was
characterized both in vitro, with cotranslation assays, and in vivo, with
microinjected Xenopus laevis oocytes. In wheat germ extracts, coat protein
synthesis was constant when a fixed amount of full-length CP RNA was
cotranslated with increasing concentrations of competitor globin mRNA.
However, translation of CP RNA lacking the 3' UTR decreased significantly
under competitive conditions. RNA stabilities were equivalent. In X. laevis
oocytes, which are translationally saturated and are an inherently
competitive translational environment, full-length CP RNA assembled into
large polysomes and coat protein synthesis was readily detectable.
Alternatively, CP RNA lacking the 3' UTR sedimented as small polysomes, and
little coat protein was detected. Again, RNA stabilities were equivalent.
Site-directed mutagenesis was used to localize RNA sequences or structures
required for competitive translation. Since the CP RNA 3' UTR has an
unusually large number of AUG nucleotide triplets, two AUG-containing sites
were altered in full-length RNA prior to oocyte injections. Nucleotide
substitutions at the sequence GAUG, 20 nucleotides downstream of the coat
protein termination codon, specifically reduced full-length CP RNA
translation, while similar substitutions at the next AUG triplet had little
effect on translation. The competitive influence of the 3' UTR could be
explained by RNA- protein interactions that affect translation initiation
or by ribosome reinitiation at downstream AUG codons, which would increase
the number of ribosomes committed to coat protein synthesis.
Copyright © 1997, American Society for Microbiology
Identification of a competitive translation determinant in the 3' untranslated region of alfalfa mosaic virus coat protein mRNA
Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge 02139, USA.
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