Mapping of mitochondrial mRNA termini in Arabidopsis thaliana: t-elements contribute to 5′ and 3′ end formation

With CR-RT-PCR as primary approach we mapped the 5' and 3' transcript ends of all mitochondrial protein-coding genes in Arabidopsis thaliana. Almost all transcripts analyzed have single major 3' termini, while multiple 5' ends were found for several genes. Some of the identified 5' ends map within promoter motifs suggesting these ends to be derived from transcription initiation while the majority of the 5' termini seems to be generated post-transcriptionally. Assignment of the extremities of 5' leader RNAs revealed clear evidence for an endonucleolytic generation of the major cox1 and atp9 5' mRNA ends. tRNA-like structures, so-called t-elements, are associated either with 5' or with 3' termini of several mRNAs. These secondary structures most likely act as cis-signals for endonucleolytic cleavages by RNase Z and/or RNase P. Since no conserved sequence motif is evident at post-transcriptionally derived ends, we suggest t-elements, stem-loops and probably complex higher order structures as cis-elements for processing. This analysis provides novel insights into 5' and 3' end formation of mRNAs. In addition, the complete transcript map is a substantial and important basis for future studies of gene expression in mitochondria of higher plants.