Germ cells are the only cells that transmit genetic information to the next generation, and they therefore must be prevented from differentiating inappropriately into somatic cells 1 . A common mechanism by which germline progenitors are protected from differentiation-inducing signals is a transient and global repression of RNA polymerase II (RNAPII)-dependent transcription 1 . In both Drosophila and Caenorhabditis elegans embryos, the repression of messenger RNA transcription during germ cell specification correlates with an absence of phosphorylation of Ser 2 residues in the carboxy-terminal domain of RNAPII (hereafter called CTD) 2 , a critical modification for transcriptional elongation 3 . Here we show that, in Drosophila embryos, a small protein encoded by polar granule component (pgc) is essential for repressing CTD Ser 2 phosphorylation in newly formed pole cells, the germline progenitors. Ectopic Pgc expression in somatic cells is sufficient to repress CTD Ser 2 phosphorylation. Furthermore, Pgc interacts, physically and genetically, with positive transcription elongation factor b (P-TEFb), the CTD Ser 2 kinase complex, and prevents its recruitment to transcription sites. These results indicate that Pgc is a cell-type-specific P-TEFb inhibitor that has a fundamental role in Drosophila germ cell specification. In C. elegans embryos, PIE-1 protein segregates to germline blastomeres, and is thought to repress mRNA transcription through interaction with P-TEFb 4-7 . Thus, inhibition of P-TEFb is probably a common mechanism during germ cell specification in the disparate organisms C. elegans and Drosophila. pgc RNA, a component of the Drosophila germ plasm 8 , has been implicated in the repression of CTD Ser 2 phosphorylation in early pole cells 9,10 . However, the mechanism underlying pgc-mediated transcriptional repression has been unknown. Initial characterization of its nucleotide sequence suggested that pgc RNA acts as a non-coding RNA 8 . Nevertheless, we have noticed that an AUG triplet, beginning at nucleotide 117 of the 0.7-kilobase (kb) transcript, is in a favourable context to serve as a translation initiation site 8 . Completion of the Drosophila genomic sequence revealed an error in our original manual sequencing in a region where a strong stem structure can form (an additional C exists between nucleotides 178 and 179). In the revised pgc sequence, the open reading frame (ORF) beginning at nucleotide 117 is capable of encoding a 71-amino-acid polypeptide (Fig. 1a). We found that this polypeptide
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