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Molecular and Cellular Biology, September 1998, p. 5140-5147, Vol. 18, No. 9
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Initiation of Protein Synthesis in Mammalian Cells with
Codons Other Than AUG and Amino Acids Other Than
Methionine
Harold J.
Drabkin and
Uttam L.
RajBhandary*
Department of Biology, Massachusetts
Institute of Technology, Cambridge, Massachusetts 02139
Received 29 April 1998/Returned for modification 10 June
1998/Accepted 12 June 1998
Protein synthesis is initiated universally with the amino acid
methionine. In Escherichia coli, studies with anticodon
sequence mutants of the initiator methionine tRNA have shown that
protein synthesis can be initiated with several other amino
acids. In eukaryotic systems, however, a yeast initiator tRNA
aminoacylated with isoleucine was found to be inactive in initiation in
mammalian cell extracts. This finding raised the question of whether
methionine is the only amino acid capable of initiation of protein
synthesis in eukaryotes. In this work, we studied the activities, in
initiation, of four different anticodon sequence mutants of human
initiator tRNA in mammalian COS1 cells, using reporter genes carrying
mutations in the initiation codon that are complementary
to the tRNA anticodons. The mutant tRNAs used are aminoacylated with
glutamine, methionine, and valine. Our results show that in the
presence of the corresponding mutant initiator tRNAs, AGG and GUC can
initiate protein synthesis in COS1 cells with methionine and valine,
respectively. CAG initiates protein synthesis with glutamine
but extremely poorly, whereas UAG could not be used to initiate protein
synthesis with glutamine. We discuss the potential applications of the
mutant initiator tRNA-dependent initiation of protein synthesis with
codons other than AUG for studying the many interesting aspects of
protein synthesis initiation in mammalian cells.
*
Corresponding author. Mailing address: Department of
Biology, 68-671A, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139. Phone: (617) 253-4702. Fax:
(617) 252-1556. E-mail: bhandary{at}wccf.mit.edu.
Molecular and Cellular Biology, September 1998, p. 5140-5147, Vol. 18, No. 9
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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