Abstract
During the transition from the maternal to the zygotic developmental program, the expression of genes important for pattern formation or cell cycle regulation changes dramatically. Rapid changes in gene expression are achieved in part through the control of mRNA stability. This report focuses on bicoid, a gene essential for formation of anterior embryonic structures in Drosophila melanogaster. bicoid mRNA is synthesized exclusively during oogenesis. Here, we show that bicoid mRNA stability is regulated. While bicoid mRNA is stable in retained oocytes, in unfertilized eggs, and during the first 2 h of embryogenesis, specific degradation is activated at cellularization of the blastoderm. To identify cis-acting sequences required for bicoid mRNA's regulated stability, fusions between bicoid and genes producing stable mRNAs were introduced into the Drosophila germ line by P-element-mediated transformation. The analysis of the fusion mRNAs identified a bicoid instability element (BIE) contained within a 43-nucleotide sequence immediately following the stop codon. The BIE is sufficient to destabilize the otherwise-stable ribosomal protein A1 mRNA and is separable from the previously identified bicoid mRNA localization signals and from the "nanos response element." Similar mechanisms may regulate a class of developmentally important maternal genes whose mRNA has a temporal profile similar to that of bicoid.
