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Figure 1


Fig. 1. Experimental approach. (A) Cartoon illustrating the principles that underly ESI. (B) Conventional EM image. HeLa cells were permeabilised, polymerases allowed to extend transcripts in BrUTP, BrRNA indirectly immunolabelled with gold particles, and 70 nm sections stained with uranyl acetate. The nucleolus and heterochromatin are heavily stained with the heavy metal, to appear black-on-white (as many electrons are scattered and so not detected). Inset, three gold particles (marking nascent BrRNA) also scatter electrons to appear dark. (C) ESI (P maps) of unstained cells (i.e. lacking uranyl acetate). Left, cell prepared as in B, but without staining. Right, control that was fixed directly, and was not permeabilised, immunolabelled or stained. P-rich structures, such as heterochromatin, appear white-on-black, and permeabilisation has little effect on morphology. Scale bars: 1 µm (inset in B, 50 nm).





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