Transcriptional and post-transcriptional regulation of maternal and zygotic cytoskeletal tropomyosin mRNA during Drosophila development correlates with specific morphogenic events.

We have characterized maternal and zygotic cytoskeletal tropomyosin mRNA expression during Drosophila embryogenesis using in situ hybridization to endogenous cytoskeletal tropomyosin mRNA and a cytoskeletal tropomyosin promoter/beta-galactosidase-encoding fusion gene mRNA in transgenic flies. A 2.0-kb maternal cytoskeletal tropomyosin mRNA is synthesized in the nurse cells and transported into the oocyte during oogenesis. During early embryogenesis, this mRNA becomes localized to the pole cell region and then to the cortex during the cellular blastoderm stage. In later embryos it is localized to the ventral and cephalic furrows and extending germ band. The major zygotic mRNA is 2.4 kb and is first detected at gastrulation. In early embryos, this mRNA is expressed in the invaginating anterior and posterior midgut, the transverse furrows, and the amnioserosa, all regions of the embryo undergoing intense cellular movement, and in later embryos, predominantly in the gut, brain, and epidermis. A transgene construct containing 1.2 kb of 5' cytoskeletal tropomyosin promoter sequences driving expression of Escherichia coli beta-galactosidase and the cytoskeletal tropomyosin 3' untranslated region in transgenic flies has the same distribution of maternal and zygotic transcripts throughout oogenesis and embryonic development as the endogenous transcripts. A transgene containing the 3' untranslated region from either the hsp70 gene or the SV40 early genes, on the other hand, does not express maternal RNA. Furthermore, none of the transgenes expressed in the follicle cells, suggesting that expression in these cells is under different transcriptional control. Our results indicate that maternal and zygotic cytoskeletal tropomyosin mRNAs are localized to specific regions of the developing embryo, particularly in regions of the embryo undergoing cell movement and invagination. Furthermore, the synthesis, transport, and accumulation of this RNA is under transcriptional and post-transcriptional control.