Abstract
According to the tRNA punctuation model, the mitochondrial genome (mtDNA) of mammals and arthropods is transcribed as large polycistronic precursors that are maturated by endonucleolytic cleavage at tRNA borders and RNA polyadenylation. Starting from the newly sequenced mtDNA of Ixodes ricinus and using a combination of mitogenomics and transcriptional analyses, we found that in all currently-sequenced tick lineages (Prostriata, Metastriata and Argasidae) the 3'-end of the polyadenylated nad1 and rrnL transcripts does not follow the tRNA punctuation model and is located upstream of a degenerate 17-bp DNA motif. A slightly different motif is also present downstream the 3'-end of nad1 transcripts in the primitive chelicerate Limulus polyphemus and in Drosophila species, indicating the ancient origin and the evolutionary conservation of this motif in arthropods. The transcriptional analyses suggest that this motif directs the 3'-end formation of the nad1/rrnL mature RNAs, likely working as a transcription termination signal or a processing signal of precursor transcripts. Moreover, as most regulatory elements, this motif is characterized by a taxon-specific evolution. Although this signal is not exclusive of ticks, making a play on words it has been named "Tick-Box", since it is a check mark that has to be verified for the 3'-end formation of some mt transcripts, and its consensus sequence has been here carefully characterized in ticks. Indeed, in the whole mtDNA of all ticks, the Tick-Box is always present downstream of nad1 and rrnL, mainly in non-coding regions (NCRs) and occasionally within trnL(CUN). However, some metastriates present a third Tick-Box at an intriguing site--inside the small NCR located at one end of a 3.4 kb translocated region, the other end of which exhibits the nad1 Tick-Box--hinting that this motif could have been involved in metastriate gene order rearrangements.
