Fig. 2. mRNA transcription and its repression during germ cell specification. (A) Steps of RNA transcription. (a) Initiation: the pre-initiation complex [consisting of RNAPII (blue) and the general transcriptional factors (GTFs, pink)] assembles at the promoter. (b) Promoter clearance: Cdk7 (orange) in the TFIIH complex phosphorylates Ser5 in the CTD repeats (green circles), allowing the polymerase to clear the promoter and recruit capping enzymes. (c) 5' capping and pausing: shortly after initiation, RNAPII is paused by the action of negative factors [DISF (DRB sensitivity-inducing factor) and NELF (negative elongation factor)]. P-TEFb (positive transcription elongation factor b) phosphorylates CTD Ser2 (green circles), DISF and NELF, promoting the dissociation of NELF and the conversion of DSIF into a positive elongation factor, leading to productive elongation (d). CTD Ser2 phosphorylation also promotes recruitment of mRNA-processing enzymes. (B-D) Repression of mRNA transcription by germline proteins. (B) In one- to two-cell stage C. elegans embryos, maternally loaded OMA-1 and OMA-2 compete with TAF-12 for binding to the GTF TAF-4, keeping it sequestered in the cytoplasm. (C) In the P₂-P₄ blastomeres, PIE-1 interacts with the Cyclin T subunit of P-TEFb, blocking its interaction with the CTD and Ser2 phosphorylation. PIE-1 also inhibits Ser5 phosphorylation by an unknown mechanism. (D) In Drosophila pole cells, Pgc binds to the Cdk9 subunit of P-TEFb, and prevents P-TEFb recruitment to chromatin (broken gray arrow). The mechanism that blocks Ser5 phosphorylation is not known.