REGULATION OF ZYGOTIC GENE-EXPRESSION IN DROSOPHILA PRIMORDIAL GERM-CELLS

Activation of the zygotic genome is a prerequisite for the transition from maternal to zygotic control of development. The onset of zygotic transcription has been well studied in somatic cells, but evidence; su ggests that it is controlled differently in the germline. In Drosophil a, zygotic transcription in the soma has been detected as early as one hour after egg laying (AEL) [1]. In the germline, general RNA synthes is is not detected until 3.5 hours AEL (stage 8) [2] and poly(A)contai ning transcripts are not observed in early germ cell nuclei [3]. Howev er, rRNA gene expression has been demonstrated at this time [4]. There fore, either there is a general, low level activation of the genome in early germ cells, or specific classes of genes, such as those transcr ibed by RNA polymerase (RNAP) II, are repressed. We addressed this iss ue by localizing the potent transcriptional activator Ga14-VP16 to the germline, and we find that Gal4-VP16-dependent gene expression is rep ressed in early germ cells. In addition, localization of germ plasm to the anterior reveals that it is sufficient to repress Bicoid-dependen t gene expression, Thus, even in the presence of known transcriptional activators, RNAP II dependent gene expression is actively repressed i n early germ cells. Furthermore, once the germ cell genome is activate d, we find that vasa is expressed specifically in germ cells. This exp ression does not require proper patterning of the soma, indicating tha t it is likely to be controlled by the germ plasm. (C) Current Biology Ltd.