AN 18-BP ELEMENT IN THE 5' UNTRANSLATED REGION OF THE DROSOPHILA BETA-2 TUBULIN MESSENGER-RNA REGULATES THE MESSENGER-RNA LEVEL DURING POSTMEIOTIC STAGES OF SPERMATOGENESIS

During Drosophila spermatogenesis transcriptional activity is mainly r estricted to premeiotic stages. Translation during sperm morphogenesis , however, proceeds for several days, requiring a high stability for m RNAs translated postmeiotically. We studied expression of the Drosophi la beta2 tubulin gene, which is expressed solely in the male germ line from the primary spermatocyte stage onwards. Cis-acting elements invo lved in the regulation of mRNA levels were investigated in transgenic fly strains. In adult testes, mRNA amounts from beta2-lacZ fusion gene s in the presence of an 18-bp AT-rich element, termed beta2DE1, are el evated about threefold. The element is present at about the same posit ion in the 5' untranslated regions of the beta2 tubulin genes of the d istantly related species Drosophila melanogaster and Drosophila hydei. Changing the position of the element on the mRNA reduces the stabiliz ing effect, while inversion of the beta2DE1 abolishes its function. Th e element also acts in a combination with the beta1 tubulin transcript ion start site, and the beta2UE1, which is required to achieve tissue- specific expression. In all experiments done, the comparison of premei otic with postmeiotic stages strongly implies that this element is inv olved in regulating mRNA stability. This mRNA stabilizing element acts in a position-independent manner and also on a heterologous mRNA, sho wing its autonomous functional activity.