The relative importance of transcriptional and post transcriptional regulation of Drosophila chorion gene expression during oogenesis

To determine the relative roles of transcriptional and post-transcriptional events in establishing the temporal pattern of chorion gene expression in Drosophila, we have examined chorion gene transcription, RNA accumulation, and protein synthesis in follicles of selected pre-, early-, and late-choriogenic stages. Chorion gene transcription was assayed in follicle cell nuclei by nuclear run-on reactions. For the s15, s16, s18, s36, and s38 chorion genes, the periods of intense transcription are as predicted from the dynamics of RNA accumulation and protein synthesis, indicating that these genes are primarily regulated at the transcriptional level. In contrast, gene s19 appears subject to post-transcriptional control at stage 14, when transcription rates are substantially higher than predicted from the observed RNA levels. Transcription of regions between the clustered and tandemly oriented chorion genes was also examined. In contrast to many RNA polymerase II transcribed genes, for the s18 and s36 chorion genes run-on transcription appears to terminate within about 100 base pairs downstream of the polyadenylation sites, corroborating previous reports based on electron microscopy of s36 [Osheim et al., EMBO J 5:3591-3596, 1986].