Fig. 7. A model for the roles of POS-1 and SPN-4 in the glp-1 mRNA translation. (Top) SCR: the 66-base spatial control region within the glp-1 mRNA 3' UTR. TCR: the 125-base temporal control region within the glp-1 mRNA 3' UTR. In this model, POS-1 binds the SCR with its second CCCH finger. SPN-4 binds the TCR. POS-1 suppresses and SPN-4 activates the translation. SPN-4 may also inhibit the POS-1 function by direct binding. (Bottom) POS-1 is abundant in the posterior blastomere (right) but present at a much lower level in the anterior one (left), while SPN-4 is abundant in both. We propose that the translation of maternal glp-1 mRNA is suppressed in the posterior half of the embryo by the POS-1 repressor activity, while it is turned on in the anterior half, where the concentration of SPN-4 and its activating function predominates.