Figure 6.

Fibrillarin is targeted to the nucleolar domain even in the absence of zygotic RNA pol I transcription. The fluorescent in situ transcription assay by immunolabeling of incorporated Br-UTP coupled to immunolabeling of fibrillarin was performed on nuclei isolated 12 h after fertilization from control embryos (A) and embryos treated with 0.5 μg/ml of actinomycin D (B). A, Sites of transcription were labeled in nuclei isolated from control embryos (a) and colocalized with fibrillarin in the nucleolar domain (b). This transcription was RNA pol I transcription because it was not inhibited by incubation of nuclei in 100 μg/ml of α-amanitin before and during incorporation of Br-UTP (c). B, When whole embryos were treated with 0.5 μg/ml of actinomycin D during development, no RNA pol I transcription was detected (e). Although RNA pol I activity was inhibited, fibrillarin was clustered in the nucleolar domain (f) as in control nuclei. Bars, 10 μm.