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We report that Vg1, a maternal mRNA localized to the vegetal hemisphere of frog eggs, encodes a member of the transforming growth factor-beta (TGF-beta) family of proteins. Furthermore, we show that Vg1 mRNA is distributed to presumptive endodermal cells after fertilization. Previous studies had shown that the vegetal end of a frog egg produces a signal that induces the overlying animal pole cells to form mesodermal tissue. More recently it has been shown that fibroblast growth factor (FGF) and TGF-beta can participate in the induction of muscle. Together, these results lead us to propose that the formation of mesoderm during frog development is specified by the products of localized maternal mRNAs, including Vg1.
Figure 1. Diagram and Sequence of the Vgl Transcript
(A) Diagram of the Vgi transcript. Restriction sites are as follows: P Pstl; B, BarnHI; Bg, Bglll; Bt, BstEll; K, Kpnl. The open reading frame of the
Vgl transcript is shown below, with the region believed to direct the protein to the endoplasmic reticulum marked as a signal sequence, and the
region of homology to TGF-3 is indicated. (B) Nucleotide sequence of the Vgl transcript including some upstream sequence. The start site of transcrip
tion is marked “0” and is accurate to within 2 bases.
Figure 2. Mapping the 5’ End of the Vgl Transcript by Primer Extension
M. pWi322 Hpall markers; lane 1, extension products of primer plus
oocyte RNA; lane 2, extension products of primer alone; lane 3, primer
alone. The extension of Vgl RNA is indicated by the arrow.
Figure 3. Comparison of Vgl Protein with Selected Members of the TGF-b Gene Family
The probable processing sites are enclosed in boxes, the conserved cysteine residues are marked by asterisks, and the possible glycosylation site
conserved between Vgl and dpp is indicated by the bar. In each case only the carboxyl end of a much larger protein is shown.
Figure 4. In Situ Hybridization of Vgl Probe to Oocytes, Eggs, and
Paraffin sections of an albino oocyte (A), an unfertilized egg (B and D),
and an early blastula embryo (C)were hybridized with 32P-labeled Vgl
(A, B, and C) or 32P-labeled histone H4 (D) antisense RNA probes.
The hybridization grains (white) show the location of Vgl or histone
mRNAs. Autoradiographic exposure was for 10 days (A and B), 2 weeks
(C), or 2 days(D). In each case, the animal (ectodermal) pole is at the
top and the vegetal (endodermal) pole is at the bottom. GV is the
oocyte’s nucleus or germinal vesicle.