Fig. 5. Nucleolar localization of CTCF is dependent on the transcription from rDNA. UR61 cells were transfected with the full-length CTCF fused with the GFP (GFP-CTCF) and either left untreated (A) or treated 24 hours post-transfection with 0.05 µg/ml actinomycin D (ActD) for 1 hour to specifically inhibit RNA pol I (B). Cells were fixed, immunostained for UBF, fibrillarin or B23 as indicated. Nucleic acids were visualized with propidium iodide (PI). The localization of the GFP-CTCF fusion protein (a,d,g,j; green channel), the markers for the different nucleolar compartments (b,e,h,k; red channel) and the merged images (c,f,i,l) are shown. Confocal images show that GFP-CTCF is released from the nucleolus upon inhibition of transcription by RNA pol I. Bar, 20 µm in a; 10 µm in b-l.