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Journal of Bacteriology, May 2000, p. 2886-2892, Vol. 182, No. 10
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Initiation of Protein Synthesis in Saccharomyces cerevisiae Mitochondria without Formylation of the Initiator tRNA

Yan Li,1 William B. Holmes,2 Dean R. Appling,2 and Uttam L. RajBhandary1,*

Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139,1 and Department of Chemistry and Biochemistry, Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, Texas 787122

Received 12 January 2000/Accepted 17 February 2000

Protein synthesis in eukaryotic organelles such as mitochondria and chloroplasts is widely believed to require a formylated initiator methionyl tRNA (fMet-tRNAfMet) for initiation. Here we show that initiation of protein synthesis in yeast mitochondria can occur without formylation of the initiator methionyl-tRNA (Met-tRNAfMet). The formylation reaction is catalyzed by methionyl-tRNA formyltransferase (MTF) located in mitochondria and uses N10-formyltetrahydrofolate (10-formyl-THF) as the formyl donor. We have studied yeast mutants carrying chromosomal disruptions of the genes encoding the mitochondrial C1-tetrahydrofolate (C1-THF) synthase (MIS1), necessary for synthesis of 10-formyl-THF, and the methionyl-tRNA formyltransferase (open reading frame YBL013W; designated FMT1). A direct analysis of mitochondrial tRNAs using gel electrophoresis systems that can separate fMet-tRNAfMet, Met-tRNAfMet, and tRNAfMet shows that there is no formylation in vivo of the mitochondrial initiator Met-tRNA in these strains. In contrast, the initiator Met-tRNA is formylated in the respective "wild-type" parental strains. In spite of the absence of fMet-tRNAfMet, the mutant strains exhibited normal mitochondrial protein synthesis and function, as evidenced by normal growth on nonfermentable carbon sources in rich media and normal frequencies of generation of petite colonies. The only growth phenotype observed was a longer lag time during growth on nonfermentable carbon sources in minimal media for the mis1 deletion strain but not for the fmt1 deletion strain.

* Corresponding author. Mailing address: Department of Biology, Rm. 68-671, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139-4307. Phone: (617) 253-4702. Fax: (617) 252-1556. E-mail: bhandary{at}mit.edu.

Journal of Bacteriology, May 2000, p. 2886-2892, Vol. 182, No. 10
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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