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Molecular and Cellular Biology, December 1999, p. 8505-8512, Vol. 19, No. 12
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Non-AUG Initiation of AGAMOUS mRNA Translation in
Arabidopsis thaliana
José Luis
Riechmann,
Toshiro
Ito, and
Elliot M.
Meyerowitz*
Division of Biology, California Institute of
Technology, Pasadena, California 91125
Received 11 June 1999/Returned for modification 3 August
1999/Accepted 18 August 1999
The MADS box organ identity gene AGAMOUS
(AG) controls several steps during Arabidopsis
thaliana flower development. AG cDNA contains an open
reading frame that lacks an ATG triplet to function as the translation
initiation codon, and the actual amino terminus of the AG protein
remains uncharacterized. We have considered the possibility that
AG translation can be initiated at a non-AUG codon. Two
possible non-AUG initiation codons, CUG and ACG, are present in the
5' region of AG mRNA preceding the highly conserved MADS
box sequence. We prepared a series of AG genomic constructs in which these codons are mutated and assayed their activity in phenotypic rescue experiments by introducing them as transgenes into
ag mutant plants. Alteration of the CTG codon to render
it unsuitable for acting as a translation initiation site does not affect complementation of the ag-3 mutation in transgenic
plants. However, a similar mutation of the downstream ACG codon
prevents the rescue of the ag-3 mutant phenotype.
Conversely, if an ATG is introduced immediately 5' to the disrupted ACG
codon, the resulting construct fully complements the
ag-3 mutation. The AG protein synthesized in vitro by
initiating translation at the ACG position is active in DNA binding and
is of the same size as the AG protein detected from floral tissues,
whereas AG polypeptides with additional amino-terminal residues do not
appear to bind DNA. These results indicate that translation of
AG is initiated exclusively at an ACG codon and prove
that non-AUG triplets may be efficiently used as the sole translation
initiation site in some plant cellular mRNAs.
*
Corresponding author. Mailing address: Division of
Biology 156-29, California Institute of Technology, Pasadena, CA 91125. Phone: (626) 395-6889. Fax: (626) 449-0756. E-mail:
meyerow{at}cco.caltech.edu.

Present address: Mendel Biotechnology, Hayward, CA
94545.
Molecular and Cellular Biology, December 1999, p. 8505-8512, Vol. 19, No. 12
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
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