Topologies of N6-adenosine methylation (m6A) in land plant mitochondria and their putative effects on organellar gene expression

Abstract

Mitochondria serve as major sites of ATP production and play key roles in many other metabolic processes that are critical to the cell. As relicts of an ancient bacterial endosymbiont, mitochondria contain their own hereditary material (i.e. mtDNA, or mitogenome) and a machinery for protein biosynthesis. The expression of the mtDNA in plants is complex, particularly at the post-transcriptional level. Following transcription, the polycistronic pre-RNAs undergo extensive modifications, including trimming, splicing and editing, before being translated by organellar ribosomes. Our study focuses on N6-methylation of adenosine ribonucleotides (m6A-RNA) in plant mitochondria. m6A is a prevalent modification in nuclear-encoded mRNAs. The biological significance of this dynamic modification is under investigation, but it is widely accepted that m6A mediates structural switches that affect RNA stability and/or activity. Using m6A-pulldown/RNA-seq (m6A-RIP-seq) assays of Arabidopsis and cauliflower mitochondria, we provide information on the m6A-RNA landscapes in Arabidopsis thaliana and Brassica oleracea mitochondria. The results show that m6A targets different types of mitochondrial transcripts, including known genes, mtORFs, as well as non-coding (transcribed intergenic) RNA species. While ncRNAs undergo multiple m6A modifications, N6-methylation of adenosine residues with mRNAs seem preferably positioned near start codons and may modulate their translatability.