Fig. 2. Visualization of psoralen signals on polytene chromosomes. (A) Biotinylated psoralen signals were detected with Alexa 488-labeled streptavidin (light green color). (B) DAPI signals (blue color). (C) Merged image. Arrows indicate representative interbands and puffs. In some loci, intense psoralen signals were observed only on the side margins of an interband or puff (arrowheads). (D) Biotinylated psoralen signals disappeared when the photo-crosslinking step was omitted. (E) DAPI image of (D). (F) Biotinylated psoralen signals were resistant to RNase treatment. (G) DAPI image of (F). Bars, 10 µm.