Restriction of the Xenopus DEADSouth mRNA to the primordial germ cells is ensured by multiple mechanisms

DEADSouth mRNA encoding the RNA helicase DDX25 is a component of the germ plasm in Xenopus laevis. We investigated the mechanisms underlying its specific mRNA expression in primordial germ cells (PGCs). Based on our previous findings of several microRNA miR-427 recognition elements (MREs) in the 3' untranslated region of the mRNA, we first examined whether DEADSouth mRNA was degraded by miR-427 targeting in somatic cells. Injection of antisense miR-427 oligomer and reporter mRNA for mutated MREs revealed that DEADSouth mRNA was potentially degraded in somatic cells via miR-427 targeting, but not in PGCs after the mid-blastula transition (MBT). The expression level of miR-427 was very low in PGCs, which probably resulted in the lack of miR-427-mediated degradation. In addition, the DEADSouth gene was expressed zygotically after MBT. Thus, the predominant expression of DEADSouth mRNA in the PGCs is ensured by multiple mechanisms including zygotic expression and prohibition from miR-427-mediated degradation.