Primary sequence of wheat mitochondrial 5S ribosomal ribonucleic acid: functional and evolutionary implications

Using the procedures of Donis-Keller et al. [Donis-Keller, H., Maxam, A. M., & Gilbert, W. (1977) Nucleic Acids Res. 4, 2527--2538 (1977)] and Peattie [Peattie, D. A. (1979) Proc. Natl. Acad. Sci. U.S.A. 76, 1760--1764], we have determined the nucleotide sequence of wheat mitochondrial 5S ribosomal ribonucleic acid (rRNA). This sequence [Formula: see text] is the first to be reported for a plant mitochondrial RNA. A highly conserved region (underlined) readily identifies the molecule as a structural homologue of other 5S rRNAs, as do potential base-paired regions which are characteristic of all known (prokaryotic, chloroplast, eukaryotic cytosol) 5S rRNA sequences. However, when assessed in terms of those structural features which distinguish prokaryotic from eukaryotic 5S rRNAs, wheat mitochondrial 5S rRNA cannot be classified readily as one or the other but instead displays characteristics of both types. In addition, the mitochondrial 5S rRNA has several unusual features, including (i) a variable number (two to three) of A residues at both the 5' and 3' ends, (ii) a unique sequence (CGACC, italic) in place of the prokaryotic sequence (CGAAC) which has been postulated to interact with aminoacyl-tRNA during translation, and (iii) a novel sequence, AUAUAUAU, immediately following the highly conserved sequence. In terms of overall primary sequence, wheat mitochondrial and cytosol 5S rRNAs seem to be slightly more divergent from each other than either is from Escherichia coli 5S rRNA, with which they are about equally homologous. From these observations, we propose that wheat mitochondrial 5S rRNA represents a distinct class of 5S rRNA. Our observations raise a number of questions about the evolutionary origin and functional role(s) of plant mitochondrial 5S rRNA.