**4.1. Atlantic cod mitochondrial mRNAs**

Similar to that of human cells, 11 mature mRNAs were readily expressed from the Atlantic cod mitogenome [47]. There are, however, some minor differences in mitochondrial mRNA maturation and modification between human and Atlantic cod. Mapping of the 5′ ends in mitochondrial mRNAs by pyrosequencing revealed that 10 of the 11 mRNAs contain no, or very short (1–2 nt), 5′ untranslated regions (UTRs) [47]. The only exception is the 5' UTR of the COII mRNA, which contained a short hairpin structure. In Atlantic cod and all other Gadidae species, this hairpin structure is capped by a GAAA tetra-loop (**Figure 3B**) [47]. GAAA tetra-loops are known to frequently participate in long-range RNA:RNA tertiary interactions [48].

Most Atlantic cod mRNAs lack 3' UTRs, but the COI mRNA has a 3' UTR of 76 nt corresponding to the complete mirror sequence of tRNASer(UCN) (**Figure 3B**) [47]. A very similar 3' UTR (72 nt) has been reported in the human COI mRNA [49], indicating a conserved role in vertebrates. The 3' UTR of the ND5 mRNA is highly variable in length in vertebrates but is lacking completely in Atlantic cod [40, 47]. However, the closely related Gadidae species *Pollachius virens* (Saithe) contains an ND5 mRNA 3' UTR of 16 nucleotides [47]. In humans, mitochondrial mRNAs contain short polyA tails of 40–50 adenosines at their 3′ ends [40, 45]. PolyA tails were identified in all mRNAs, except for ND6 mRNA [40], and seven UAA termination codons were created in the human mitochondria by polyA posttranscriptional editing [50]. Similarly, all mitochondrial mRNAs (except the ND6 mRNA) were found to be polyadenylated in Atlantic cod, and six UAA termination codons were generated by polyA addition [47].
