Gemline development regulation by maternally localized factor PEM

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Suppression of zygotic transcription in early embryonic germline cells is tightly linked to its separation from the somatic lineage. Many invertebrate embryos utilize localized maternal factors that are successively inherited by the germline cells for silencing the germline. Germline quiescence has also been associated with the underphosphorylation of Ser2 of RNA polymerase II. Using an ascidian Halocynthia roretzi, we identified a first deuterostome example of a maternally localized factor, PEM, which represses germline transcription through Ser2 underphosphorylation. Our results suggest that non-homologous proteins, PEM, Pgc of Drosophila, and PIE-1 of C. elegans, repress germline gene expression through analogous functions: by keeping Ser2 underphosphorylated through binding to the P-TEFb complex, a kinase that phosphorylates Ser2. The present study provides an interesting example of evolutionary constraint on how a mechanism of germline silencing can evolve in diverse animals.