RT Journal Article SR Electronic T1 RNA polymerase II activity is located on the surface of protein-rich transcription factories JF Journal of Cell Science JO J. Cell Sci. FD The Company of Biologists Ltd SP 1999 OP 2007 DO 10.1242/jcs.027250 VO 121 IS 12 A1 Eskiw, Christopher H. A1 Rapp, Alexander A1 Carter, David R. F. A1 Cook, Peter R. YR 2008 UL http://jcs.biologists.org/content/121/12/1999.abstract AB We used electron spectroscopic imaging to map nucleoplasmic transcription sites in human cells at unprecedented resolution. HeLa cells were permeabilised, nascent transcripts were extended in BrUTP by ∼40 nucleotides and the resulting BrRNA immunolabelled with gold particles before structures were viewed. Nascent RNA is almost invariably associated with polymorphic and nitrogen-rich (but phosphorus-poor) structures with a diameter of ∼87 nm and mass of 10 MDa (calculated by reference to nucleosomes with known numbers of phosphorus and nitrogen atoms). Structures with similar atomic signatures and diameters were observed using correlative microscopy and in unpermeabilised cells. Our results are consistent with RNA synthesis occurring on the surface of these huge protein-rich transcription factories.