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Development 2010 Feb 01;1374:651-60. doi: 10.1242/dev.038554.
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Repression of zygotic gene expression in the Xenopus germline.

Venkatarama T , Lai F , Luo X , Zhou Y , Newman K , King ML .

Primordial germ cells (PGCs) in Xenopus are specified through the inheritance of germ plasm. During gastrulation, PGCs remain totipotent while surrounding cells in the vegetal mass become committed to endoderm through the action of the vegetal localized maternal transcription factor VegT. We find that although PGCs contain maternal VegT RNA, they do not express its downstream targets at the mid-blastula transition (MBT). Transcriptional repression in PGCs correlates with the failure to phosphorylate serine 2 in the carboxy-terminal domain (CTD) of the large subunit of RNA polymerase II (RNAPII). As serine 5 is phosphorylated, these results are consistent with a block after the initiation step but before the elongation step of RNAPII-based transcription. Repression of PGC gene expression occurs despite an apparently permissive chromatin environment. Phosphorylation of CTD-serine 2 and expression of zygotic mRNAs in PGCs are first detected at neurula, some 10 hours after MBT, indicating that transcription is significantly delayed in the germ cell lineage. Significantly, Oct-91, a POU subclass V transcription factor related to mammalian Oct3/4, is among the earliest zygotic transcripts detected in PGCs and is a likely mediator of pluripotency. Our findings suggest that PGCs are unable to respond to maternally inherited endoderm determinants because RNAPII activity is transiently blocked while these determinants are present. Our results in a vertebrate system further support the concept that one strategy used repeatedly during evolution for preserving the germline is RNAPII repression.

PubMed ID: 20110330
PMC ID: PMC2827618
Article link: Development
Grant support: [+]

Species referenced: Xenopus
Genes referenced: birc5 bix1.3 h3-3a h4c1 nanos1 nodal1 pgat pgc polr2a sox17b.2 tbxt vegt
Antibodies: H3f3a Ab1 H3f3a Ab3 H3f3a Ab5 H3f3a Ab6 Hist1h4a Ab1 Hist1h4a Ab5 Methylcytosine Ab1 Nanos1 Ab1 Polr2a Ab2 Polr2a Ab3 Polr2a Ab4

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References [+] :
Asaoka, Maternal Nanos regulates zygotic gene expression in germline progenitors of Drosophila melanogaster. 1998, Pubmed