Van Doren M, Williamson A L, Lehmann R
Skirball Institute, NYU Medical Center, New York 10016, USA.
Curr Biol. 1998 Feb 12;8(4):243-6. doi: 10.1016/s0960-9822(98)70091-0.
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 suggests that it is controlled differently in the germline. In Drosophila, zygotic transcription in the soma has been detected as early as one hour after egg laying (AEL) [1]. In the germline, general RNA synthesis is not detected until 3.5 hours AEL (stage 8) [2] and poly(A)-containing transcripts are not observed in early germ cell nuclei [3]. However, rRNA gene expression has been demonstrated at this time [4]. Therefore, either there is a general, low level activation of the genome in early germ cells, or specific classes of genes, such as those transcribed by RNA polymerase (RNAP) II, are repressed. We addressed this issue by localizing the potent transcriptional activator Gal4-VP16 to the germline, and we find that Gal4-VP16-dependent gene expression is repressed in early germ cells. In addition, localization of germ plasm to the anterior reveals that it is sufficient to repress Bicoid-dependent gene expression. Thus, even in the presence of known transcriptional activators, RNAP II dependent gene expression is actively repressed in early germ cells. Furthermore, once the germ cell genome is activated, we find that vasa is expressed specifically in germ cells. This expression does not require proper patterning of the soma, indicating that it is likely to be controlled by the germ plasm.
合子基因组的激活是从母体控制向合子控制发育转变的先决条件。合子转录的起始在体细胞中已得到充分研究，但有证据表明在生殖系中其调控方式有所不同。在果蝇中，早在产卵后一小时（AEL）就已在体细胞中检测到合子转录[1]。在生殖系中，直到AEL 3.5小时（第8阶段）才检测到一般RNA合成[2]，并且在早期生殖细胞核中未观察到含poly(A)的转录本[3]。然而，此时已证明有rRNA基因表达[4]。因此，要么早期生殖细胞中存在基因组的一般低水平激活，要么特定类别的基因，如由RNA聚合酶（RNAP）II转录的基因，受到抑制。我们通过将强效转录激活因子Gal4-VP16定位于生殖系来解决这个问题，我们发现Gal4-VP16依赖性基因表达在早期生殖细胞中受到抑制。此外，将生殖质定位于前部表明它足以抑制依赖于Bicoid的基因表达。因此，即使存在已知的转录激活因子，RNAP II依赖性基因表达在早期生殖细胞中也被积极抑制。此外，一旦生殖细胞基因组被激活，我们发现vasa在生殖细胞中特异性表达。这种表达不需要体细胞的正确模式形成，表明它可能受生殖质控制。
