**4. Mitochondrial transcriptome**

Vertebrate mitochondrial transcriptomes have mainly been studied in human cells and tissues [36, 37]. Mature mitochondrial RNAs are generated from three polycistronic transcripts initiated within CR from two H-strand promoters (HSP1 and HSP2 ) and a single L-strand promoter (LSP) (**Figure 3A**) [36, 38–40]. The HSP1 -specific transcript is highly abundant and generates mtSSU rRNA, mtLSU rRNA, as well as tRNAVal and tRNAPhe [41, 42]. HSP1 -specific tRNAs have recently been proposed to perform a second role as a mitochondrial rRNA, substituting the lacking 5S rRNA in vertebrate mitochondrial ribosomes [43, 44]. While tRNAVal appears associated with the mitochondrial ribosomes in human and rat, tRNAPhe has been identified in porcine and bovine [45].

Ten H-strand-specific mRNAs are posttranscriptionally processed from the HSP<sup>2</sup> transcript, together with 13 tRNAs and the two rRNAs (**Figure 3A**) [46]. Most HSP2 mRNAs are monocistronic, but two of the mRNAs are bicistronic (ND4/4 L mRNA and ATPase8/6 mRNA). Finally, the L-strand-specific transcript gives rise to the ND6 mRNA and eight tRNAs (**Figure 3A**).
