Impact of genomic environment on mitochondrial rps7 mRNA features in grasses

Mapping of wheat mitochondrial mRNA termini and comparison with breakpoints in DNA homology among plants

Mitochondrial DNA rearrangements occur very frequently in flowering plants and when close to genes there must be concomitant acquisition of new regulatory cis-elements. To explore whether there might be limits to such DNA shuffling, we have mapped the termini of mitochondrial mRNAs in wheat, a monocot, and compared them to the known positions for counterpart genes in the eudicot Arabidopsis. Nine genes share homologous 3' UTRs over their full-length and for six of them, the termini map very close to the site of wheat/Arabidopsis DNA rearrangements. Only one such case was seen for comparisons of 5' UTRs, and the 5' ends of mRNAs are typically more heterogeneous than 3' termini. Approximately half of the thirty-one wheat mitochondrial transcriptional units are preceded by CRTA promoter-like motifs, and of the potential stem-loop or tRNA-like structures identified as candidate RNA processing/stability signals near the 5' or 3' ends, several are shared with Arabidopsis. Comparison of the mitochondrial gene flanking sequences from normal fertile wheat (Triticum aestivum) with those of Aegilops kotschyi which is the source of mitochondria present in K-type cytoplasmic male sterile wheat, revealed six cases where mRNAs are precluded from sharing full-length homologous UTRs because of genomic reorganization events, and the presence of short repeats located at the sites of discontinuity points to a reciprocal recombination-mediated mode of rearrangement.

Potential role of tRNAs in wheat and Lolium mitochondrial rps7 transcript processing

The wheat mitochondrial gene for ribosomal protein S7 exhibits multiple transcripts that share the same 3′ terminus but range in overall length from 3.4 to 0.7 kb because of 5′end-maturation events. The longest detectable precursor RNA maps precisely to the 3′ end of a chloroplast-origin tRNA-Phe gene, consistent with it providing signals for endonucleolytic cleavage. Steady-state levels of precursor RNAs were seen to be lower in seedlings than in germinating embryos, although the degree of editing within untranslated regions (UTRs) was higher in seedlings. In another grass, Lolium multiflorum Lam., rps7 displays transcripts of 1.3 and 0.7 kb, and although the distal 5′ UTRs are unrelated in sequence to those of wheat, the 5′ terminus of the longer transcript also maps to a tRNA gene, in this case the native mitochondrial-type tRNA-Ser. Our findings illustrate the plasticity of plant mitochondrial transcriptional units and the recruitment of chloroplast-origin sequences for the expression of mitochondrial genes.