RELATIONSHIP BETWEEN EXPRESSION OF SERENDIPITY ALPHA AND CELLULARISATION OF THE DROSOPHILA EMBRYO AS REVEALED BY INTERSPECIFIC TRANSFORMATION

A dramatic reorganization of the cytoskeleton underlies the cellularis ation of the syncytial Drosophila embryo. Formation of a regular netwo rk of acto-myosin filaments, providing a structural framework, and pos sibly a contractile force as well, appears essential for the synchrono us invagination of the plasma membrane between adjacent nuclei. The se rendipity alpha (sry alpha) gene is required for this complete reorgan ization of the microfilaments at the onset of membrane invagination. W e compare here the structure and expression of sry alpha between D. ps eudoobscura, D. subobscura and D. melanogaster. Interspersion of evolu tionarily highly conserved and divergent regions is observed in the pr otein. One such highly conserved region shows sequence similarities to a motif found in proteins of the ezrin-radixin-moesin (ERM) family. F our 7-13 bp motifs are conserved in the 5' promoter region; two of the se are also found, and at the same position relative to the TATA box, in nullo, another zygotic gene recently shown to be involved in cellul arisation. The compared patterns of expression of D. melanogaster sry a and nullo, and D. pseudoobscura sry alpha reveal a complex regulatio n of the spatiotemporal accumulation of their transcripts. The D. pseu doobscura sry alpha gene is able to rescue the cellularisation defects associated with a complete loss of sry alpha function in D. melanogas ter embryos, even though species-specific aspects of its expression ar e maintained. Despite their functional homologies, the D. melanogaster and D. pseudoobscura sry alpha RNAs have different subcellular locali sations, suggesting that this specific localization has no conserved r ole in targeting the sry alpha protein to the apical membranes.