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.

The mitochondrial genomes of flowering plants are highly recombinogenic and this can lead to altered transcriptional units, even between closely related species. We are interested in the effects that DNA rearrangements have on the generation of mature mRNAs, and to this end we have determined the termini of mitochondrial S7 ribosomal protein (rps7) mRNAs from selected grasses, using circularized-RT-PCR. Although the rps7 mRNAs show a similar size of about 750 nt by northern hybridization analysis and have virtually identical 3' UTRs, their 5' terminal extremities differ among plant species, and this is attributable to genome rearrangements in some but not all cases. In wheat, rice, and barley, the 5' ends are homogeneous for each plant but map to non-homologous sites among the three species. In contrast, the rye, brome and Lolium 5' ends are quite heterogeneous in length even though they are located within conserved genomic regions. Comparative sequence analysis suggests that certain grass lineages have retained an ancestral organization upstream of rps7 that includes a 170-bp block homologous to sequences preceding several other mitochondrial genes, whereas others have undergone independent rearrangements at a recombination-prone site. Our analysis of mature rps7 transcripts revealed two non-silent RNA edits within the coding sequences, and also editing at several sites within the conserved 5' and 3' UTR regions in these plants, raising the possibility of their role in rps7 expression at the post-transcriptional level. Taken together, our observations illustrate the dynamic nature of upstream regulatory cis-elements for mitochondrial rps7 mRNA production in contrast to conservative 3' end-formation signals, during evolution in grasses.

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