Xenopus laevis Vg1 mRNA undergoes both localization
and translational control during oogenesis. Vg1 protein does
not appear until late stage IV, after localization is complete.
To determine whether Vg1 translation is regulated by cytoplasmic
polyadenylation, the RACE-PAT method was used. Vg1 mRNA has
a constant poly(A) tail throughout oogenesis, precluding a role
for cytoplasmic polyadenylation. To identify cis-acting
elements involved in Vg1 translational control, the Vg1 3′
UTR was inserted downstream of the luciferase ORF and in vitro
transcribed, adenylated mRNA injected into stage III or stage
VI oocytes. The Vg1 3′ UTR repressed luciferase translation
in both stages. Deletion analysis of the Vg1 3′ UTR revealed
that a 250-nt UA-rich fragment, the Vg1 translational element
or VTE, which lies 118 nt downstream of the Vg1 localization
element, could repress translation as well as the full-length
Vg1 3′ UTR. Poly(A)-dependent translation is not necessary
for repression as nonadenylated mRNAs are also repressed, but
cap-dependent translation is required as introduction of the
classical swine fever virus IRES upstream of the luciferase
coding region prevents repression by the VTE. Repression by
the Vg1 3′ UTR has been reproduced in Xenopus
oocyte in vitro translation extracts, which show a 10–25-fold
synergy between the cap and poly(A) tail. A number of proteins
UV crosslink to the VTE including FRGY2 and proteins of 36,
42, 45, and 60 kDa. The abundance of p42, p45, and p60 is
strikingly higher in stages I–III than in later stages,
consistent with a possible role for these proteins in Vg1
translational control.