**5.1. Atlantic cod mitochondrial long noncoding RNAs**

Two lncRNAs (lncCR-H and lncCR-L) have been identified and investigated in Atlantic cod mitochondria (**Figure 4**) [10, 18, 47]. Both lncRNAs were found to be polyadenylated but transcribed from opposite strands within the CR [10]. We showed that the Atlantic cod lncCR-L has a mutation rate and an expression level corresponding to that of Complex I mRNAs [10, 18, 47]. The lncCR-L apparently corresponds to the 7S RNA in human mitochondria [52], and recently we showed that lncCR-L is differentially expressed in a human cancer-matched cell line pair [56].

The lncCR-H was found to be highly variable in sequence and structure, both between and within Atlantic cod specimens [10, 18]. A schematic overview of the lncCR-H RNA is presented in **Figure 4**. Here, the noncoding T–P spacer is present at the 5′ end and includes two potential RNA hairpin structures. The T–P spacer domain is followed by a mirror tRNAPro, before entering the HTR array motifs. The HTR copy numbers vary between 2 (80 bp)

**Figure 4.** Schematic view of CR and corresponding noncoding RNAs in Atlantic cod. tRNA genes (tRNAThr, tRNAPro, tRNAPhe), T–P spacer, HTR (heteroplasmic tandem repeat array), TAS (termination associated sequence), and CSB2 (conserved sequence box 2) are indicated. The H-strand-specific lncCR-H is located at the 5′ domain of CR and is the precursor of two enriched small RNAs (above CR map). The L-strand-specific lncCR-L is located at the central domain

and more than 8 (>320 bp) [5, 14, 15, 18], rendering lncCR-H highly variable in size. Finally, lncCR-H terminates in a short polyA tail at TAS. Thus, lncCR-H has apparently no fixed length in Atlantic cod mitochondria and varies in size between approximately 300 and 500 nt. Interestingly, the TAS motif consists of a perfect palindromic sequence motif (**TTTATACAT**ATGTATAAA). We found lncCR-L to terminate with a polyA tail at the same site as lncCR-H but on the opposite strand [10].
