| Summary: | Adenosine deaminases acting on RNA (ADAR) catalyze the hydrolytic deamination of adenosine (A) to produce inosine (I) in double-stranded RNA substrates. A-to-I RNA editing has increasingly broad physiological significance in development, carcinogenesis, and environmental adaptation. <i>Perionyx excavatus</i> is an earthworm with potent regenerative potential; it can regenerate the head and tail and is an advantageous model system to investigate the molecular mechanisms of regeneration. During RNA sequencing analysis of <i>P. excavatus</i> regenerates, we identified an <i>ADAR</i> homolog (<i>Pex-ADAR</i>), which led us to examine its spatial and temporal expression to comprehend how <i>Pex-ADAR</i> is linked to regeneration. At first, in domain analysis, we discovered that <i>Pex-ADAR</i> only has one double-stranded RNA-binding domain (dsRBD) and a deaminase domain without a Z-DNA-binding domain (ZBD). In addition, a comparison of the core deaminase domains of <i>Pex-ADAR</i> with those of other ADAR family members indicated that <i>Pex-ADAR</i> comprises the conserved three active-site motifs and a glutamate residue for catalytic activity. <i>Pex-ADAR</i> also shares 11 conserved residues, a characteristic of ADAR1, supporting that <i>Pex-ADAR</i> is a member of ADAR1 class. Its temporal expression was remarkably low in the early stages of regeneration before suddenly increasing at 10 days post amputation (dpa) when diverse cell types and tissues were being regenerated. In situ hybridization of <i>Pex-ADAR</i> messenger RNA (mRNA) indicated that the main expression was observed in regenerating muscle layers and related connective tissues. Taken together, the present results demonstrate that an RNA-editing enzyme, <i>Pex-ADAR</i>, is implicated in muscle redifferentiation during earthworm regeneration.
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