Fig. 7. POS-1 competes with SPN-4 for binding to nos-2 3'UTR. (A) Electrophoretic mobility shift of the radiolabeled 200 bp nos-2 3'UTR incubated with GST:SPN-4 alone (lane 1), GST:POS-1 alone (lane 2) or with increasing concentration of GST:POS-1 at a constant concentration of GST:SPN-4 (lanes 3-7). No protein was added to RNA in lane 8. Lanes 1 and 3-7 contain 2 µl of GST:SPN-4 per lane. Amounts of GST:POS-1 in lane 2 are 4 µl and in lanes 3-7 are 2, 4, 6, 8 and 10 µl, respectively. (B) Representative examples of 16- and 28-cell embryos immunostained with anti-SPN-4 and anti-POS-1 antibodies. (C) Bar graph showing average POS-1: SPN-4 ratios obtained by quantitation of immunofluorescence signals from 12 embryos for the indicated stages.