Mitochondrial Introns as Mobile Genetic Elements: the Role of Intron-Encoded Proteins
Introns of organelle genes share distinctive RNA secondary structures that allow their classification into two known families. These structures are believed to play an essential role in splicing, and members of both structural classes have recently been shown to perform self-splicing reactions in vitro. In lower eukaryotes, many structured introns also contain long internal open reading frames (ORFs), which are able to code for hydrophilic proteins.
Several properties of self-splicing structured introns suggest that they resemble mobile genetic elements, even though no actual transposition event involving these introns has yet been found. We report here on the characterization of two intron-encoded proteins that strongly support this attractive idea. First, we show that the class I intron of the 21S ribo-somal RNA (rRNA) gene of Saccharomyces cerevisiae omega strains (r1 intron) encodes a specific transposase. This protein has been partially purified from Escherichia coli cells that overexpress it from an artificial universal code equivalent to the rl intronic ORF. The omega transposase shows a double-strand endonuclease activity in vitro. This activity creates a 4-bp staggered cut with 3′ OH overhangs within a specific sequence of the 21S rRNA gene of omega strains. It is precisely within this sequence that the rl intron inserts by a duplicative transposition. Second, we report on the synthesis, in E. coli, of a putative reverse transcriptase encoded by the class II intron of the cytochrome b gene of Schizosaccharomyces pombe. This synthesis was obtained from E. coli expression vectors, using the class II intronic ORF linked to an artificial initiator sequence.
As further support of the idea that structured introns are mobile, we show, from a systematic screening of introns in various yeast species, that the r1 intron has transposed into the ATPase subunit 9 gene of Kluyveromyces fragilis. Structural features observed at the new intron homing site may be relevant to the transposition event.
KeywordsMobile Genetic Element ATPase Subunit Structure Intron Mitochondrial Intron Duplicative Transposition
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- 8.Church, G., and W. Gilbert (1980) Yeast mitochondrial intron products required in trans for RNA splicing. In Mobilization and Reassembly of Genetic Information, D.R. Joseph, J. Schultz, W.A. Scott, and R. Werner, eds. Academic Press, Inc., New York, pp. 379–395.Google Scholar
- 12.Dujon, B., and A. Jacquier (1983) Organization of the mitochondrial 21S rRNA gene in Saccharomyces cerevisiae: Mutants of the peptidyl transferase centre and nature of the omega locus. In Mitochondria 1983, R.J. Schweyen, K. Wolf, and F. Kaudewitz, eds. W. de Gruyter and Co., Berlin, New York, pp. 389–403.Google Scholar
- 13.Dujon, B., M. Bolotin-Fukuhara, D. Coen, J. Deutsch, P. Netter, P.P. Slonimski, and L. Weill (1976) Mitochondrial genetics. XI. Mutations at the mitochondrial locus omega affecting the recombination of mitochondrial genes in Saccharomyces cerevisiae. Mol. Gen. Genet. 143: 131–165.PubMedCrossRefGoogle Scholar
- 14.Dujon, B., G. Cottarel, L. Colleaux, M. Betermier, A. Jacquier, L. d’Auriol, and F. Galibert (1985) Mechanism of integration of an intron within a mitochondrial gene: A double strand break and the trans-posase function of an intron encoded protein as revealed by in vivo and in vitro assays. In Achievements and Perspectives in Mitochondrial Research, F. Palmieri, ed. Elsevier, Amsterdam, pp. 215–225.Google Scholar
- 28.Matsura, E.T., J.M. Domenico, and D.J. Cummings (1986) Curr. Genet. (in press).Google Scholar
- 29.Maxam, A.M., and W. Gilbert (1980) Sequencing end-labeled DNA with base specific chemical cleavages. In Methods in Enzymology, Vol. 65, L. Grossman and K. Moldave, eds. Academic Press, Inc., New York, pp. 499–560.Google Scholar
- 40.Shapiro, J. (1983) Mobile Genetic Elements, Academic Press, Inc., New York.Google Scholar