DEVELOPMENTAL REGULATION OF BICOID MESSENGER-RNA STABILITY IS MEDIATED BY THE FIRST 43 NUCLEOTIDES OF THE 3'-UNTRANSLATED REGION

During the transition from the maternal to the zygotic developmental p rogram, the expression of genes important for pattern formation or cel l cycle regulation changes dramatically. Rapid changes in gene express ion are achieved in part through the control of mRNA stability. This r eport focuses on bicoid, a gene essential for formation of anterior em bryonic structures in Drosophila melanogaster. bicoid mRNA is synthesi zed exclusively during oogenesis. Here, we show that bicoid mRNA stabi lity is regulated. While bicoid mRNA is stable in retained oocytes, in unfertilized eggs, and during the first 2 h of embryogenesis, specifi c degradation is activated at cellularization of the blastoderm, To id entify cis-acting sequences required for bicoid mRNA's regulated stabi lity, fusions between bicoid and genes producing stable mRNAs were int roduced into the Drosophila germ line by P-element-mediated transforma tion. The analysis of the fusion mRNAs identified a bicoid instability element (BIE) contained within a 43-nucleotide sequence immediately f ollowing the stop codon. The BIE is sufficient to destabilize the othe rwise-stable ribosomal protein Al mRNA and is separable from the previ ously identified bicoid mRNA localization signals and from the ''nanos response element.'' Similar mechanisms may regulate a class of develo pmentally important maternal genes whose mRNA has a temporal profile s imilar to that of bicoid.